Decade old thread but perhaps details below will help someone else.
Going down freeway in 5th and then 'no 5th', no noise, all else OK.
Pulled tranny (after driving for a few weeks without 5th) snap ring on main shaft at back end of tranny popped out of its slot, and drive gear on output shaft slipped off the spline onto bare section of shaft.
Could have fixed just removing the xfer case and driving on a new snap ring, wold be a 15 in job on a 2 WD if you had the 1" snap ring on hand
I had a habit of using 5th to brake, that forces slanted gear OFF the spline (drive forces against main bearing) Lots of steep hills here so 5th for braking seemed wise, until now. Will not use 5th for downhill braking any longer.
BTW, '84 NWC tranny.