Recently bought an '82 Camaro Z-28 and after replacing the muffler, the car has been great. Except for one problem that just started.

When I start the car in the morning or after it has been sitting for a while, it starts right up and everything is golden. But, when I put it in drive or reverse, the back-end shakes pretty violently until I get past 5 mph. Once I am past 5 mph, everything is fine until I stop. Then the back ends shakes/vibrates again until I start moving again and everything is fine again.

I don't think it's the transmission because it seems to be shifting fine. And here's the thing that is throwing me off: after 5 mins of driving or after a short time sitting, the vibrations/shakes disappear. So when the car warms up, everything is fine. For example, I can be sitting at a red light after 5 mins of driving and the car sounds and feels great.

I hope I explained that clear enough. Any idea what this could be? I don't want to take it to the shop if I can do something myself. Any help would be greatly appreciated. Thanks.

 

I dont think its drivetrain related. The warming up part, only at idle, and the fact it started after u replaced the exhaust pointed me towards your exhaust. It is very tight where it goes over the panhard support brace. My guess is when your engine is cold it vibrates a bit from side to side at idle causing the exhaust to hit the brace or something else in the rear of the car. Does this sound like a possibility to you or am i way off?