Transfer case motor...which part for 99 yukon?

[DaveEye]

I'm about to order a new transfer case motor for my wife's '99 Yukon SLT. The switch lights up and I hear the click under the dash, but no 4WD, so I'm figuring it's the motor. Anyway, at rockauto.com they list 4 different parts. How can I tell which one I need for my wife's Yukon? Thanks in advance!