When I first accelerate from a dead stop, I can feel my engine shift (only if I hit the gas slightly hard). A mechanic suggested replacing my motor mount. My guess is that it will be the front motor mount that needs to be replaced? Has anyone changed motor mounts on 92 Deville 4.9? Can this be accomplished with a standard 3 ton jack and a couple of jack stands or do I need additional items to lift the engine for the mount replacement? Is there a specific way to tell which ones are bad?

It's a fairly straightforward job. I believe a 13mm and 15mm will take care of it. Jack up the car and place jackstands behind the subframe on the body's "frame rails". Take loose the front subrame bolts and, with a floor jack, lower the subframe slowly until you have enough clearance to access the mount. Place jackstands on subframe to hold this position. Take loose the motor mount bolts. Place a block of wood on your floor jack and carefully jack the engine on the oil pan. Simple R&R from this point on. The top motor mount nuts an be a little awkward with the AC compressors position, just takes a little patience.