Low-speed "clunk" 1-2 shift, A-904

Something I also forgot to mention is that the car will shudder noticeably when backing up slowly (but not quickly). How do I check the u-joints and rear end backlash? It's the stock 7-1/4" rear end.

the shudder could be the front clutch or the rear band slipping .

to check the backlash grab the driveshaft with the trans in neutral , apply the emergency brake and block a tire if the rear is on the ground and see how much playt there is and what is moving . the 78 lebaron I had made alot of noise when putting it in gear , higher fluid pressure and a stack up of clearance makes it loud when going in gear either reverse or forward ..