check your transmission fluid level. sounds like you're a bit low. Then figure out why it's low. probably a leaking seal. I've heard goode and bad about tranny flushes. That would have to be your call. If you do get a flush make sure it's done by a trans shop and not some guy in a service station or Bobs friendly corner garage. Trans shops do transmissions and not much else. They would know the tricks for yours. But I think you are just low on fluid.
OR.......check your U-joints on all your shafts. You might have one trhying to let go.