4X4 problem

[Ohiosonomaguy]

Got a 2001 Sonoma 4x4 ... only about 54k miles. The 4WD is acting up .. seems only when its cold or wet or both. I can start off driving in 2WD, and then midway through my trip, I will look over and its shifted itself into 4HI. It won't let me shift back to 2WD - except once the truck has sat for a bit in the garage, I can shift it back. Sometimes when I start it up there are NO lights that come on (its the electronic on the dash) and then it will finally come up to 4HI. I can sometimes shift it into 2WD, but like this morning on the way to work as I'm crusing down the highway, it will just go into 4HI..... any suggestions on what the problem may be? And am I looking at big bucks to get something like this fixed? Any help appreciated!!