Clunk! Pop! What could cause this?? (Suspension related probably)

Ok so sometimes when i'm slowing down i feel and hear like a thud almost like my rear end is catching up with my car. Its not violent but it is easily noticable. Also, when i am stopped or going really slow, then i start to take off (slowly even) i hear a couple of pops which also are from the rear of the car. I'm going to jack the car up this weekend or maybe tomorrow to check things out and i wondered if you guys might know what to look for. I think the pops might be due to shot spring isolators, because i didn't plan ahead and used the old ones with my new springs when i did my suspension at the beginning of the summer. I'm also thinking maybe the clunk/thud is a loose bolt or something. I dont currently have the rear sway bar on (finally getting around to putting it on when i jack the car up), could that be it? Would the shot isolators cause it? I dont have quad shocks, do they have anything to do with it? ANY ideas are really appreciated, i HAVE to fix this, i'm afraid something is about to straight fall apart. Thanks for any help, hopefully i can fix the problem this weekend.
-E

Thanks a lot guys! The driveshaft is the other thing i was gonna check while i was under there. I'll probably be doing it tomorrow, i'll post my findings.
-E

I got under the car today to check the u-joints. I had the car in gear and tried to turn it back and forth with my hand, it has a pretty good bit of play. So would that pretty much do everything that i described in the previous post?? Thanks,
-E

I put the e-brake on and i also put the car in gear (5-spd). Then i jacked it up and got under and grabbed the the drive shaft with my hand. I tried to rotate it by hand and it had play in it. It doesn't move side to side or anything but i can rotate it by hand a small bit back and forth, it isn't tight in place. So that means the u-joints are shot right?
-e