I'm not advocating running out and spending $80-$100 on a new motor in hopes that it will fix your problem. Just saying it's one of the items that needs to be considered before assuming the transfer case needs to be opened up and repaired; which, of course, isn't cheap (cost me around $700 to have a shop rebuild mine last Spring). Unfortunately, I'm not sure how to bench test a motor.
In my own situation, as a last resort, I made the decision to take a chance by replacing the motor myself to save some money as I figured a shop would recommend a 19 yr. old motor be replaced as part of a rebuild anyway...and, they would charge me at least double what it would cost to do it myself. And, I just might have gotten lucky and solved the problem. Well, it didn't solve the problem...but, at least I knew I'd have a new motor on a newly rebuilt transfer case for a little less money spent.