I have a '92 F-150 2wd extend cab short box. My problem is that from a stop up to 30/32 mph I have a vibration that starts out strong and then gets weaker with speed until it goes away. I have a two piece drive shaft, if that helps diagnois. I don't know when it started to do this, but it has done this since I got the truck, several months ago. My guesses are u-joint, center support bearing, or possibly a broken motor mount. How do I narrow this down with out just throwing parts at it? Thanks for the help in advance.

 

Well I replaced the u-joints, and center support bearing and I still have the vibration. I did notice that the driver side rear brake is dragging fairly heavily. Is that normal since that is where the parking brake is?

 

Check your rear upper shock mounts. Are they broken? Do you have any cracked, broken or missing leafs in your springs? Check u-bolts (that attach around axle housing & bolt to springs). Are the u-bolts tight?

One other question. Doe's the transmission have a slip yoke that slides into the rear sail housing? If so, this front driveshaft is called a "happy shaft" & in some instances may require a special d/s support brg. or hanger bearing. This "happy shaft" syndrome is more prevelant on chassis that have been shortened, lengthened or modified (such as motorhomes or limos.).

I had a similiar problem arise with my 1979 F-150--ended up being a broken L/H, upper shock mount.

 

I think it has what you called the happy shaft. It is a stock short box extended cab 2wd. I replaced the u-joint that was at the tranny end of that shaft, the center support (carrier?) bearing, and the two u-joints on the back shaft. I was looking at the shocks that are on the truck and the rear shocks appear to be the original shocks (84,000 miles). They aren't broken, but don't seem to be doing much besides taking up space. Thanks for the other possible areas to look. I'm about to my wits end with this problem & I hate the thought of taking it to a mechanic just to have them tell me they couldn't find the problem, but here's your bill.