Something large is shifting under the truck during acceleration/ braking and I’m having trouble figuring out what it is.

I can slowly get moving from a stop and it won’t happen until I accelerate enough to cause it to go “thunk.” So it doesn’t seem like excessive play in driveline rotation.

Here’s what I’ve tried / done:

- Replaced the rear end recently with one that has 70k. (Diff ate itself)
- U-joints were tight. Checked during rear end job with the driveshaft out.
- Greased driveshaft slip joint.
- Replaced motor and trans mounts.
- Replaced front radius arm bushings.

When I get truck on a lift I can’t see any signs of something moving or have found any loose hardware. Any ideas?

 

Lubricate the driveline slip yokes with this stuff and it will drive like a dream
Slip yoke klunk is most likely what is happening
Good luck, have fun

Ford FG-8A driveline grease