Limited slip rears require 75w140 full synthetic lube and a 4 oz bottle of modifier. You can use any brand of fluid from anywhere, but get the modifier at the dealer. I used a gasket on mine when I did it, Autozone has them for 5 bucks. At least they do for my 8.8 axle, you have a 9.75 if you have a 5.4. The capacity is 5.5 pints, so you will need 3 quarts of fluid. Fluid is about 15 bucks a quart and I think the modifier is about a tenspot. It needs to be changed at least every 100k.

 

Ok thanks again everyone... I think I will bite the bullet this weekend and get to it... weather permitting. I looked over a couple how to's on the web and the diff fluid does seem like its pretty easy... alot of them said that you can just drain it through the drain plug or with a pump through the fill plug... what's the advantage to popping the cover off? Also what are the signs of bad diff fluid... is it similar to old engine oil... black and nasty? I think while I'm at it I might drop the drive shaft and re-grease the spline shaft...any preferences on type of grease to use? Will I need to clean all the old grease off? Thanks again for everyone's help... it really is a good world we live in when people are willing to help without expecting something in return....
Also... thinking to use Mobil 1 gear oil... any thoughts on that? I know that royal purple is supposed to be great... but wow look at the price.

 

The jack stands just make the job easier if you cant jack it up just lower the spare tire so you have room to kinda sit under the rear of the truck. As jbrew stated if you have a 9.75 rear end which i do you have to use rtv sealant. You are suppose to let the sealant set before you fill the differential with fluid for at least a few hours.

 

Sounds like U joints to me. Also if you have a 2 piece driveshaft the carrier can cause that as well

 

thanks Nick.... I was thinking that dropping the spare might give me a bit more room. I don't think i have the 9.75 differential... mine i think is 8.8. so on the mobil 1 is that a decent gear oil?
OGmackPLRcool, thanks for your post too... i too was thinking u joint, but i tried to shake the crap out of it and there didn't seem to be any play...can they be bad without play? Its a one piece shaft so no carrier bearing. so i'm inclined to try the diff fluid like these guys are saying... I still plan on greasing the spline slip shaft, but still not sure what type of grease to use... also I'm wondering if i can grease it without lowering the drive shaft... maybe just remove the rubber boot... I'm not sure... like I said, this is my first rear wheel drive and truck so still figuring some things out.

 

5.4L = 9.75
4.6L = 8.8

Yes, Mobil 1 is fine, just don't forget the friction modifier = 4 oz's. 4 - 6 is good.

RTV - Permatex Black and what nick said.

When that fluid breaks down, these trucks can shake so bad , it can force you off the road. It happens quite a bit.

BTW- Product called TransTune or something like (Trans something, TransX maybe ?) that is one of the friction modifiers and/or additives at third party parts stores. I've used it before, it's good.

White and blue bottle.

You can grease the slip yoke or spline (lightly), but personally, I wouldn't touch the shaft or joints if she's tight and not giving you problems, that's me. A dry spline will cause a bump and cluck vs a vibration anyway. Ford Factory greasless U-joints should last 200,000 miles +, they really did something right there. I have over a quater million miles on the joints on my 4x4 and pull a car trailer.

Also - The 9.75 has 12 bolts on the cover.

glc - covers the rest.

 

There is no drain plug - you have to pop the cover. Use a rag to wipe out the bottom of the housing when the cover is off.

Autozone lists a Fel-Pro gasket for the 9.75 rear, it's $10.99 and special order.

 

I suppose you could get one, mine didn't have one factory. Ford used RTV as well. So I figured that's the best way to go if the manufacturer chose that route. RTV is around 5 bucks.

zamunda - Make sure your breather tube is still there (axle, left side, goes straight up and connects to the bed), you can loose them over time or they can split right at the connection which invites water into your rear gears.

 

i agree with the breather tube didn't even think of that mine was cracked at the bottom and i just got a new tube from home depot its much cheaper....let us know how your fluid change turns out.

 

ok... I'll be sure to check that tube...
I think that I'll probably just go with the rtv and skip the gasket...what do you guys use to scrape off the old rtv... i know you have to be careful not to scratch and gouge the mating surfaces...
think I'll probably use the transx modifier too... easier to just get all the stuff at kragen or autozone than to go all the way to the dealer. and it looks like crc makes that transx and from what I know they make pretty good products. As for filling... is it possible to just use a funnel and pour in the new fluid... or will i need some kind of hand pump?

 

I use a vac tool, but you can just squeeze that stuff in there. The bottles the fluid and modifier come in make that possible, just squeeze lol.

A gasket scraper. Looks like s screw driver with a razor blade end. They sell cheap ones at the store that work. Most have a stash of extra blades in the handle.

 

ok... i just pulled up a picture of gear lube bottles on autozone's website... your comment of squeezing it in makes alot more sense now... I was just thinking a regular oil bottle and was thinking you were out of your head!! is it better to put a bottle of the oil in first then the modifier or modifier first... or doesn't matter? And it is 75w140 right? haven't looked at the tag on the diff yet, but I think that's what the manual says.

 

Yea, forget the tag, they take 75/140 Full Syn 100% for the rear gear. Doesn't matter on the modifier , it will mix

 

Squirt some oil in, then pour the modifier into the oil bottle and continue squirting.

 

Well before i ran to autozone to get the stuff for the change today... i decided to pop the fill plug and look at the oil and check the breather tube... etc. I popped the breather tube off the axle...and blew through it... make sure there was no clogs. It was clear.... it is attached up at the bed through a hole and has a cap or something on it. the tube itself looks ok. I took the fill plug out... and from what i understand the fluid should slightly dribble out... it didn't... it looks like the fluid might be a quarter inch maybe 3/8 below the fill hole. Maybe that is due to the truck not being perfectly level... the oil looked ok... clearish color but did have air bubbles in it... ,but i had just driven the truck about 5 miles
I looked around the cover and there is no dripping, but at about 3 or 4 bolts at the mating edge there is moist gunk/dirt/whatever... could that be signs of a slow seep? Also the cover is rusted up pretty good, the paint is flaking... is that a problem?
Thanks guys.
Also back of fill plug had gunk sludge and super fine filings... from what I know that's normal