rear end troubles

it is probably something simple like just the u-joints or something

Yeah lol.

I never heard of this "bang" noise comming from the rear. I just now if your going to slow in a low gear, you'll get a vibration feeling/noise untill it has enough strength.

i have never heard of that test.

I would have to agree that if the u-joints are wore that can cause some slack in the driveline resulting in the noise you are talking about. It can also have slack from worn gears. I have that problem on my '85. The rearend has about 200K or so on the stock gears, and i can get a slight pop when i'm in 5th and mash on it, but that's because the gears are so wore the backlash has increased because of them wearing out. I took the differential cover off and inspected them and they were fine, just a little too much backlash. I suppose this could eventually destroy them by them clashing together eventually, but they have been fine for 3 years now that way, this includes launches at the dragstrip and just not being easy on it in general.

I'd say unless it's wining real loud or popping when you take off from a light or something, and the u joints are good, then i'd probably start to worry.