2001 Ford Ranger 4 wheel drive

First I would check behind the dash is a 4x4 relay if you hear it clicking then it might be fine but maybe need replaced but id have to say the 4x4 motor on transfercase is blown out or stuck maybe try removing it and cleaning or something their is also a fuse for the 4x4 that could cause this that is under the hood in the main fuse panel im pretty shure its num 3 its a small fuse id jsut make sure they all are good but I had alot of problems with my truck bout 2 weeks ago and I noticed that when the 4x4 relay goes out that is behind steroe in dash the truck automatically switches to 4 low and their is no action at all at the switch but with the info you gave me its hard to tell but I am pretty knowledged bout 4x4 on these trucks so if you have other symtoms not mentioned I might be able to help a little more