My truck
1999 F-150 XLT O/R 4x4 SuperCab
4.6L V8
8.8" rear end with 3.55 LS
92,000 miles

I'm an adequate backyard mechanic and can do most simple repairs myself. A couple of days ago I started having some vibration problems in my truck that felt exactly like a driveshaft U-joint going bad. So I popped the driveshaft off, felt the joints and... odd... While one of the rear joint axis felt a little stiff, it didn't feel anywhere near bad enough to cause the jumping I'd felt while going down the highway. I had a somewhat cold feeling in my stomach as I replaced the joint and reinstalled the driveshaft. I took it out for a test drive and, sure enough, the vibration was still there. This pretty much eliminated the easy, cheap, U-joint fix and pointed at an expensive rear end problem.

Luckily for me, my brother is a mechanic who owns his own shop about an hour away from me. I called him up yesterday and told him what I'd found and he agreed with my diagnosis. So I drove it down to him last night. Unfortunately he had a job that was taking longer than expected, so we didn't get a chance to pull the cover and take a look as his bays were full. Instead, we had a few beers, caught up on local events and generally BS'ed for awhile. Then he gave me an older Chevy pickup to use as a loaner until he could get mine back on the road and I drove home.

Well, about 10 minutes ago he called me. He said that in 20+ years of being a mechanic, he's never seen anything like what he saw when he pulled my differential cover off. He said that the carrier itself is broken into two pieces near where the ring gear bolts on. The carrier moved around in the housing enough to actually allow the pinion to ride up over it, leaving teeth marks on the outside. The carrier bearings spun and pretty much destroyed the housing. He said the only real recourse I have is an entire axle replacement. Now, he admits that he's pretty hard on his toys and he doubts he could manage to do that to any of his trucks no matter how hard he tried.

I use my truck for daily driving to and from work. Once a year I take it out to camp which are all nice gravel roads. I very occasionally pull a single snowmobile around. I can't see any reason for this to have happened unless there was a weakness in the casting. Lucky me. :P

So now I'm wondering that, since I need to replace it anyhow, would it be better to try and look for a 9.75" rear end instead of the 8.8"? I know the 8.8" is normally a good rear end, but as long as I'm buying an entire axle anyhow, should I upgrade it to a 9.75"? I believe it was a factory option for my truck. Are there any changes/snags involved in this swap? ie - anti-lock sensor, mounting pads, brakes, etc...

I'd appreciate any thoughts on this. If I'm going to be spending big $$$$ on an entire axle replacement anyhow, I wouldn't mind going with the stronger option as long as it wouldn't be too difficult.

Thanks in advance.

 

I have the same truck. At 60,000 I replaced the first rear. At 102,000 I was told my current rear is cracked. I agree you should upgrade. I would be interested in replies from people who put an air locker of some sort to replace the standard rear. I'm tired of slipping in the ice off road. Has anyone done that? Would I have to put a locker in the front diff too? Thanks, and good luck.

 

Perhaps you should start your own thread instead of posting unrelated questions on somebody else's. I apologize as I realize that I am now contributing to this problem.

 

Has there been any sort of comparison done of the overall strength and longevity between the 8.8" and 9.75"? Has anyone ever done the swap themselves? I would really welcome some advice/links at this point since this would be the ideal time to upgrade to a 9.75" if there ever was one. There's basically nothing worth salvaging from my old axle. But I would only do the 9.75" swap if it meant increased reliability and I wouldn't have to modify/swap out too many other components. ie - brake calipers, driveshaft, etc.

 

So far I've found one link where somebody with a 1998 F-150 4x4 S.C. O/R swapped in a 9.75 from a 1999. His was equipped with drum brakes and the only change he found was that the driveshaft needed to be shortened about 1".

http://jayntguru.com/mambo/component...limitstart,12/

Anyone other info out there?

 

Okay, I've found the posts by JMC and the details of his swap. A 1" shorter driveshaft is it. Now I need to find the axle.

Does anyone know of a source for a 9.75" 3.55 limited slip axle at a decent price? The place JMC got his from doesn't have any in stock.

 

I used to work at a salvage yard and at any given time they had probably 250 rearends laying around. I left there on bad terms so I can't really help you out, but you can give them a call. WWW.affordableautosalvage.com. I inquired about doing the same swap you're talking about and they had a couple of the exact reaerend you're looking for. If they don't have any, they do have connections all over the country and they could find you one. Just don't tell them you know me lol.

 

Try http://www.car-part.com/index.htm I you have an automatic you could also look for the driveshaft from a truck with the 5.4 as it will have the 9.75 and should be the correct length. Price the used shaft vs having a shop shorten yours. There was never any 5 speed manual with a 9.75 so I had to have it shortened.

JMC