87 B-150, 225, A-904, 8-3/8" open rear.

When I let off gas and roll on level ground, or slight downgrade, there is a thud.

My first guess would be u-joints, but they are less than 2 years old, and have very little mileage.

What else should I be looking for?

Could it be the lock-up disengaging ?
When was the last time you serviced the trans?

_________________
Fast is fine but accuracy is final. You have to learn to take your time ... in a hurry
- Wyatt Earp

I would suspect U-joints, but I have had that "thunk" when the bolts have loosened up on the tranny sandwich or spool...you can also get that sound when there is excess play in the rear diff sometimes.

TCLU is electronic and disconnected.

Transmission rebuilt maybe 10 years ago (but <20,000 miles ago).

U joints my first thought,but they are really new. I will put up on jackstands Wednesday and check them.

Differential is making noise moving from a stop under certain conditions. I really need to rebuild it. This may be it just getting worse.

I did a lot of mixed driving today, and noticed it only does it when driving slow enough to not have shifted into second yet. Once in second or third: nothing. (almost like it started to shift to second, and was in no man's land between gears)

Doesn't do it when downshifting during acceleration (but up-shift after hard acceleration is a little loud).

I hope to find a transmission to rebuild over the winter and install in the spring, then go through this one well.

I never worked on a rear, and am unsure of my own ability to leave it in any condition better than I found it.

It would be nice to know if it is something I need to worry about getting stranded over.

Have you checked motor mounts and trannie mount? Being as it happens in 1st only, sounds like it needs the higher reverse torque of backing off int 1st to move things enough. Look at the rear axle mounts, rear shocks, crossmember bolts, exhaust being close to the body somewhere, etc. as well.

I check the motor mounts fairly often. (I bought a pair years ago, replaced the bad one, forgot which one I replaced, and try to notice a difference between the two so I know which one was changed already.)

The tranny mount was suspect before. I bought a new one, but never installed because that problem was elsewhere.

I'm going to jack the motor by a block under the pan to 'stretch' the mounts, which should identify the older one pretty clearly, then install the new mounts I have.

The axle mounts are good (I just installed new U-bolts when I installed junkyard springs this summer.)

rear shocks <2 yrs old.

I really doubt it is exhaust knocking on something. The pitch is way too low.

There was a little bit of freeplay @ trans mount.

I started replacing and realized that the hole through the mount was smaller than the shoulder of the bolt.

I also found that the mount to trans screws were loose and that there is a big chunk of aluminium missing at the passenger side screw hole.

I reinstalled the old mount.

I also noticed as I jacked up the van that the leaf spring and a clamp that is part of the tailpipe hanger were interfering with one another. I shoved the tailpipe towards the center of the van, bending the hanger slightly.

They no longer interfere. (I intend to remove and reinstall the exhaust pipes with walker acoustiseal at some point, now that I have good manifolds and Aussie gaskets, I want to eliminate all exhaust leaks. Ill make sure all hangs right then, and may weld something for a tailpipe hanger to eliminate the offending clamp).

I'm not sure what to do about the missing hunk of transmission.

I guess I can try to find an unrebuildable A-904 and swap tails.

Thanks for all the feedback.