Hey,
I have had this problem since I bought the truck about two years ago. When I go to reverse after driving a while and the trucks warmed up, it will engage in gear (first clunk), then the drive shaft moves violently and almost turns the tires when I have the brakes on because the clunk is so violent. Someone said I might have a little play in my pinion, but I'd like to know for sure. If anyone has any idea's, please let me know. It's really bad in reverse. You go from drive to reverse and it takes like a few seconds. The first bang happens....wait.....wait...wait.....BANG! Jerking the truck violently. It bothers the heck out of me, so any idea's would be greatly appreciated. Thanks! Oh by the way, the truck is a 1993 F150 XLT 5.0 4x4. It is equipped with the E40D Transmission.

 

Probably the u-joint where the driveshaft meets the rear diff.

Does it do the same thing in 4wd? If not, I'd say it's definitely the rear u-joint.

Cheap part, fairly easy replacement.

 

Thanks, I thought that was the problem a week ago and replaced ALL the U joints and the problem is still there. I also added a container of Lucas Transmission Fix; thinking it might of been the transmission but it is still there . Any other idea's are appreciated.

 

Have you checked codes to see what the computer has to say? With the E4OD, you'll have error codes if the tranny is at fault.

U-joints replaced... that rules out the easy fix!

I guess being 4x4, your other options could be the transfer case or the rear diff.

I'd pull codes first - there are how-to's on here, which show you how to do it with the check engine light and a paperclip or length of wire. If you don't have a manual or something similar to list code definitions, list the codes here and we can help with that.

If there are no tranny codes, you'll need a buddy to get on the ground and watch the driveshaft while you put the truck in fwd/rev/fwd/rev. You may be able to tell by timing the clunk with shaft movement if it's happening at the transfer case (before the shaft moves) or at the rear diff (after the shaft moves).

Only other question that might help narrow it down - does it do this in 4x4, or just in 2wd?

 

Here's how to read codes:
https://www.f150forum.com/f10/how-retrieve-trouble-codes-obdi-10907/

 

...I knew you'd have that link handy!

 

I would drop the drive shaft and grab the pinion yoke on the rear axle and give 'er a few twists and jerks. See if you have a lot of play on that pinion. Next, same thing with the output yoke on the t-case (tranny in park). I've had the pinion nut loosen on me and cause havoc like your discribing.