I haven't had a chance to test it out. I'm pretty sure this was it. The bolt was finger loose, not finger tight.

 

Usually the large bushing that is between the frame and the support mount bracket that is bolted to the differential are in good shape. They can sometimes get compressed at a slight angle. The TOP bushing is where I normally see the problem. They wear out and need to be replaced.

Knowing that some people have had problems in the urethane front differential mount bushings...I prefer to install then in rubber like the factory. BUT...if you want to use the polyurethane bushings. I would... at all costs.... make sure that the bushings are the SAME thickness as like the factory designed them to be in rubber. ESPECIALLY the large bushing that goes between the frame and the differential mount. I have seen some of the polyurethane bushing kits where the large bushing is thinner that the factory one and I personally do not like it.

ALSO...I would verify that the two bolts that hold the differential front mount bracket are correctly installed and tightened to the specified torque and the cotter pins installed in them where the castle nuts are. While you are there working...I would NOT assume anything.

Just to let you know that if you go and replace these bushings and the bolt....it is FUN getting the old ones out and the new ones in and the bolt installed. IT CAN BE DONE with the driveshaft still connected.....because I do it that way....but it is all about the 'angles of the dangles'. If you get frustrated and take out the driveshaft...hopefully you KNOW that you need to mark it so it goes back in the EXACT SAME WAY as you took it out.

DUB

 

Took it for a drive. The loud thud is gone. But, I still feel a small "engagement" in the rear diff when I'm getting on it from 1st to 2nd. I've got 450 ft lbs of torque hitting that rear diff. I'm wondering if it's expected.????

 

I had that much or more in my big block C3s, and the rear ends were tight under acceleration. Only when the bushings would occasionally fail did I ever have any trouble. Do what Dub said and your problem will probably be solved.

 

"Engagement"...that is so hard to decipher. This might be normal.

Not knowing what the backlash of the differential is....and if the u-joints are GOOD ONES for the pounding they will take.....and where this 'engagement' that you are feeling is actually coming from. Hard to say.

DUB

 

All new parts everywhere. New u joints, driveshaft, etc. the only thing I didn't do was replace all of the rear dif rubber's. I used all of the existing rubber bushings, etc. so the snubber bushing is original. Likewise so are the bushings between the upper rear frame crossmember and the diff.