Problem while accelerating in higher gears

[C J]

When I press the gas pedel to accelerate in higher gears my car has been
vibrating with like a rumbling thumping sound.  This only happens in fifth
gear, and sometimes 4th.  It also does it more the harder i accelerate.  So
if i barely touch the pedal it doesn't do it.  also, when it stops
increasing in speed it seems to stop.  I have no idea what it could be, any
input would be greatly appreciated.
Thanks in advance.

C.J.
96 200sx se-r