1998 GP makes "thunk" under front of console when put into reverse.It only does it in reverse

Definite possibility, the rubber inserts wear out and in some cases only half the insert gives out rather than the whole thing leaving cushion in one direction and nothing in the other. Assuming it has the 3800 it could even be the inexpensive "wishbone" mounts that go from the front head to the core support. See if you can get a second person to shift while you inspect the mounts from the side, if it turns out to be those they are extremely easy to replace just remember to do them one at a time. Get a free estimate and if you wish to do any of it yourself just reply on this post and I'll give you the Chiltons repair procedure for any work needed.