There are a bunch of threads on this.. (I know, I just searched for this last week when I had the problem).
Its problably the speed govenor (sp)? sticking.
This piece is in the transmission case (in the extension housing). To get to it, you need to drop the xfer case (not hard to do, it was harder to find the right size 12 point to pull the front/rear drive shafts off).
Once you have the xfer case out you can look into the extension housing and see the govenor. It has 2 bolts holding it on.. (its really the only thing in there that you can see, other than the output shaft)..
You take the 2 bolts off, (don't drop the govenor, and hold it together, its 4 little pieces)..
Once you, take it apart. Their is a piston type thingy (I know, technical term) that moves. If You take off the big cap/spring you can take the piston thingy out. That piece should slide on its own weight.. If it doesn't move either there is gunk on it, or it is worn. You can try some emory cloth to clean it up (don't do it too much). Aparently a new govenor assembly is cheap (<$50).
Once the pieces are clean etc.. put it back in.
From what I've read there are only 2 things that cause the govenor to stick. Dirt/crap from the tranny, and wear. In my case it was dirt since the breather lines for both the transmision and the transfer case were missing.
As for flushing the tranny, I'll let other people discuss that, but the general consensus is if the tranny has not been taken care of (e.g. 80k miles with the same fluid) don't flush it. If you keep the fluid/filter changed, then flush shouldn't hurt.
In my case, I did flush it.. I flushed 3 gallons of fluid through the transmision (I poured it in as it was coming out a cooler line I disconnected). I had 12k miles on this tranny since the last rebuild, and I've done 2 external filter changes and change the fluid in the pan once.
~Mark
