Hey, since you've tested most of the system, you're pretty much down to either the solenoids or the motor. (at least from what I understand)
If I were to hazard a guess, I'd say it's probably the motor.
If it were one of the solenoids, you'd only have a problem turning in one direction. If it's wired similar to some of the diagrams I've seen, there are 4 solenoids. 2 of the 4 solenoids engage when you choose a direction....the other two engage when you choose to go in the other direction.
There's always the chance that two of the solenoids have failed though (one for each direction), which would still produce the symptoms you've got.
Good luck! Thankfully from what I've heard, most Warn winches are repairable, so you can probably buy a new motor.