My suggestion is to find a shop or someone you know and trust that has a 4 post drive on lift. This way you can have one person in the car and one under while shifting back and forth to "Drive."

Normally the rearend will clunk and squat down a little when shifted into gear. It's due to the torque of the engine being applied and small amount of "slop" or clearances in the drivetrain. If the noise is unusually loud, like a bang. I would be worried.

Let us know if you find anything.   

If you pay attention, you will see the rear raises a bit when you put it in reverse.  The height difference is due to the engine torque applied to the rear axles vs suspension loading. This is much more noticable when the engine is running faster, such as cold fast idle.  As Sarge said, some clunk is normal, but check it out to be sure.

It sounds like the top mounting bushing is shot. If you feel a noticeable movement, I would check this first before trying anything expensive. This is the rubber mount that is at the top of the metal bracket that is bolted to the frone of the diff.

Remember my motto........

It's always something stupid, if it ain't.....it's a thousand bucks.

Good luck, and let us know how it turns out.

Alan

yes i agree with alan and would check all the diff bushings..

i doubt it would be the sombrero bushings but when you take the front bushing loose try to rock the diff.. if it rocks front to back..  they are shot..

'80-'82 differential top bushing/mounts are not the same as '63-79's. They are bolted to each frame rail vs. bolted to the crossmember above. So if those were bad... man you would have tons of racket under there. The front mount and bushing is similar to the '79 and earlier. And having 27k only. I doubt that the bushings are toast. My '81 still has the original top bushings w/100k+ on them. There still like new. The front one I replaced it only because the bushing came in the kit. Good Luck.

I need a drink now.

I would say it is your pinion bushing. Very common problem and whenever you put it into gear it clunks when they are going bad. I would replace it w/a poly bushing instead of rubber. Those bushings take a lot of abuse and the poly will out last the rubber. Here's a link. Scroll down to part number FC-09 or FC-09P.

http://www.vansteel.com/index.cfm?fuseaction=ProductList& ;SubGroup=1004