1996 Ford Ranger 4wd problem

My Ford Ranger with 45k miles is having difficulty getting into 4wd.
Sometimes it goes right in but usually, it takes a while (a minute or
so) and it seems as if it isn't really fully engaged as I get an
occasional clunk like gear teeth slipping to the next tooth.
Eventually, it seems to settle down. (It seemed to do this last
summer as well so it's probably not due to cold weather.)

It also seems slow to disengage and I usually have to back up to get
everything back to 2wd.

It has auto hubs and a switch on the dash. The indicator light goes
on as if it was engaged.

Any suggestions?