If you're feeling it IN the shifter, it's something up front - shifter is connected via cables. If it's UNDER the shifter, could be a carrier bearing? Is it vibrations or klunking. Any more details/description you can offer?
A dead motor mount would actually transfer LESS vibration as the motor would literally be almost "floating" with nothing really HOLDING it in place, but rather the mounts just there to keep it from spinning/torqueing right out of teh bay. Upper engine (driver side) and tranny are pretty easy to check. For the lowers, rev the gas and see how much teh motor twists front to back. If it a lot (more than 0.5" to 1.0" or so, I'd say the lower mounts are probably bad. The motor will have some play, but too much lan lead to damage to attached parts (axle seals, carrier bearings, etc).
EricB