91 Firebird, Rear end of the car seems to shake at stop/low speed?

I have a very weird problem with my car... 91 base Firebird, 305 TBI/700R4.

The problem is not consistent...but the rear end of my car seems to shake and jerk around a bit. When I'm at a stop, it seems that the rear end jerks a bit...once every few seconds or so. As I'm accelerating slowly, it seems like it's doing it a lot more violently (it sounds like the car is about to fall apart), once I'm up to about 30-35mph, it doesn't do it at all.

It's ONLY in the rear of the car, not in the front at all... I tried shifting into 1st or 2nd manually and it didnt do anything to help the problem...the problem also happens in Reverse...though it goes away when I'm in Park. (Haven't tried Neutral)

Any ideas as to what this might be? It sounds a lot like a transmission problem to me...I'm kinda hoping it's a rear end problem though, since I'm doing a rear end swap on Friday :)

Thanks guys...

check the trans. fluid.

if that checks out ok,i wouldnt worry about it,just go ahead and swap the rear out on friday like you were goin to do,u might find the problem while your doing the swap.

sounds like something might be loose in the rear somewhere.

does your car have disc or drum rear brakes?

he has rear drums (i know him, just replying for him so he can get a faster response)

Thanks John (MasterEvilAce), and Rice Killer :)

I'll probably need to re-do the trans fluid when I put the new axle in anyway, thanks for reminding me...if the rear end swap doesnt do it, what else would you suggest?

John and I were thinking torque converter, but it seems to be more dependent on speed, it does it even when I am stopped...

I drove around a bit tonight and it didn't seem to do it very much if at all. It mostly seems to happen when I first start up the car, but also happens other times too...

it sounds like to me something is loose.

can u hear anything rattling if u stand outside the car w/ it idleing?

get underneath and check the exhaust,that could cause vibrations.

I will try that tomorrow and let you know, but if it were the exhaust, wouldn't that just merely cause a noise...and not actually make the rear of the car shake?

Oh well, I'll give it a shot tomorrow, since it never hurts to try.

OK, I looked at it even more and I'm pretty sure it's an engine issue. The car just seems to be idling very, very rough. Nothing is rattling outside the car.

I did an oil change tonight and it helped a little, but not enough.

Any ideas? I put some fuel injector cleaner in, but I'm doubtful that will do anything to help.

MEA is coming over tomorrow to help me with my rear-end swap, I guess we'll take it for a test drive before we start.