My 87 4+3 (or diff.) has been giving me a signal for a while now. If the car has been running for a while, I can get a "clunk" on transitions, with an accompanying subtle "jump" forward. It does it reliably if I'm coasting down, and then I start to apply any throttle, no matter how lightly. Lately it does it in traffic at low RPM and low load. Last year, I had a mechanic take a look at the U-joints and differential, and he said they were fine. The brace looks tight, too. The clunk can be heard in any gear, and with OD on or off. I also get a similar clunk when manually shifting the OD, depending on the specific load conditions. It doesn't seem to matter whether I clutch the OD or not, but the clunk is heard upon engaging the clutch, not upon hitting the button. If the OD shifts down on its own, it is silent.

This thing has the earmarks of a problem downstream of the transmission to me. I've been told by one friend that a stick will always have some slop, but I don't think audible clunks and jumps are normal.

I'm looking for some guidance on diagnosis, and whether this sounds familiar to anyone. The car has 158,000 on the odometer. I do not race or autocross the car, and I only manually disengage the OD to drive in slow traffic.

 

It's either Ujoints or pinion.

The pinion has a crush fit, and if it's loose, the torque will slide the pinion in and out a bit. A worn ring/pinion set will give the same symptoms. It doesn't take much play in a u joint to do it either, especially when you have 6 of them.

I haven't heard of wheel bearings causing this.

That slop is especially bad for hard shifting, it magnifies the torque applied to the entire drivetrain and is what causes breakage.