That sounds like your transmission.
There clearly is a problem with your transmission
Having it on "PARKING"
move seat all the way fwd there will be two 16mm or 18mm bolts holding the track down easy to get to from the back seat, remove lift up and pull back
it will drive but when in reverse it wont move
reverse faults move from compression when the hanging wall moves up
blown first /reverse bands. Or clutches.
No, the seats in a 98 Cadillac Deville are all electronic so they cannot move manually. You will have to get the electrical components fixed before the seat will move.
nORTH
crankshaft
Reverse.
shift to nutral,then move lever to reverse
Get it rebuilt