Thanks nathanwright, I'd read somewhere a while ago that it could be rear upper control arms - you replaced the uppers or lowers?
I replaced my uppers, but if your uppers are shot chances are all the rear bushes are shot. Best bet would be to replace all the rear bushes (upper, lower control arms, panhard, etc) with nolathane and should be sweet.
The problem with replacing just the uppers is that the bushes will wear out quicker of the rest (lowers, etc) are shot too. So best to replace them all at once
always a good idea to check your engine/gearbox mounts too.
to check your engine mounts, start your car put it in drive have the handbrake on and your foot on the brake pedal, have the bonnet open and get a mate to take a look at the engine (or vice versa) and pulse the accel pedal (careful when doing this may cause rubber burning or send you hurtling towards your mate if you're not spot on with the brakes) if the engine lifts up and CLEARS the chassis, then your engine mount is broken.
it will be very obvious if the mount is broken, most engine mounts in older engines will be "stretched" but not broken. expect the engine to lift up a little bit but the mounts only broken if its excessive.
you'd wanna be sure it's not shuddering from the gearbox too, if it does you should be able to feel it through your auto shifter just keep your hand on it as you take off and see if theres an extreme movement there.