I want to replace the solenoid that shifts the transfer case into four high and low.

It is an electric motor and is located on the driver's side of the transfer case, tucked in next to the front U-joint. It is fairly large with a bundle of wires going to it. It also contains a switch for sensing the selector shaft position, and that switch sometimes fails. If you are parked with the driver's door open, you can hear the motor operate when you push the rocker switch. It makes a sort of a "SHHHHHH-Thunk" sound that lasts about one second either engaging or disengaging. I'm not sure what it will sound like if the "encoder" switch is bad since I've never had that problem, but I would guess that it would sound quite different. Look up service info for the GM 233 Electronic Transfer Case. The motor is pricey if new, so try logical diagnosis first, checking for power, etc. Then look for a salvage yard part. There is also an associated electronic control module which I think is located near the ECM.