Whenever I step on the gas hard there is a "thump" sound and it feels like it is coming from the transmission tunnel. I checked the torque arm, control arms, shocks, and replaced a worn tailshaft housing, but the problem remains. Any suggestions? thanks

 

Check your trans mount.

 

its your tranny mount i've gone through 2 in the last year buy a poly

 

Does the polyurathane mount cause any more vibration inside the car when compared to the stock rubber mount?

 

yes, but it's the price you pay....

i just use the stock mounts. if you have a stock horsepower car or mildly modded you will be fine for a while...if you beat on your car badly buy the better setup and deal with it.