My 1996 Z28 has a problem. Its got 72k miles on it and when it is in overdrive, and it shifts above 3,500 it makes a very severe knocking noise. It makes a DUNK DUNK sound and vibration. I'm trying to figure out if it is a transmission mount or the transmission mount. I put up on a jack from the rear end and hit the gas to were it was knocking on the street. It only jerked slightly. PLEASE HELP, SUGGESTIONS, PICTURES, DIAGRAMS, SOMETHING!!!!!

 

So, it only makes the sound and shakes on hard shifts into overdrive?
Does it have some difficulty getting into overdrive at other RPM's?
If this issue only happens on the 3-4 shift, it's hard to believe that the problem is with the trans mount. However, if you mean it clunks and vibrates on every shift when the gear selector is in OD, then it may simply be the trans mount is bad.
To check if the trans mount is bad, simply get under the car with a prybar and lift up on the tailshaft of the transmission. If you see that the rubber of the tranny mount separates or is nearly separated, the mount is bad.
It may be that the clutch bands are significantly worn and are having trouble grabbing.