Hmm.... sounds EERILY like the problem I had with my 98RS. I changed the transmission, rear diff, engine mounts, suspension, tires..... and it wound up being a FL axle.
.
My symptoms were:
- Only was really noticeable during acceleration
- Got worse as speed increased.
- Had a speed harmonic it was worse at.
Replaced the axle, and it was perfect. Took a year to figure it out
Good luck!
- dow