My buddy just got a 91' auto firebird with the 2.8l v6 it looks to be. When its shifting manually into 2nd and also when it is just in drive it makes a wierd *** sound and violently shakes, seems to be comming from the back. Its really hard to tell if its the tranny or rear end or something else. When the car is driving around in overdrive it drives perfectly fine, and when your stopped and put it in drive or over drive it takes a few seconds to engage. Please give suggestions if possible, thanks

 

check therear suspension bushings for wear e.g. control arm bushings, panhard bar. check the u-joints for play. Another thing to check for wear is the torque arm bushing which is often overlooked along with the trans mount.