I had the same problem with the drivers seat in my 97.
I was sitting IN the seat instead of ON it.
I got some of these Rugged Ridge seat covers.
Rugged Ridge Custom Fabric Seat Cover, Front (pair) Black/Tan 48024
The bottom half of the seat cover has straps which tie securely around the base of the seat and really firmed it up - feels like a new seat.
I was so impressed I immediately ordered another set of seat covers for when the original ones wore out. But I've had the originals for over 2.5 years and apart from fading, they are showing no signs of wearing out and the seat hasn't lost any of its firmness.
PS - the pic in the link is of 03-06 RR seat cover, but they are very similar.