I have an 87 IROC with a 700r4 and a 9 bolt. when accelerating hard from a stop, there is a loud thud that you can feel rock the cars before the car starts to move. I looked at the rear suspension and trans mounts and there all good. Does anyone have any hunches???

 

sure sounds like the tranny mount to me..sometimes you have to jack the tranny up to reveal a broken mount, sometimes it isnt so obvious....if thats not it..maybe a bad u-joint.