Sorry, yet another 4x4 problem..
 

Background: 2001 Jimmy 4 door 4x4 diamond edition, 4 button switch...

So, here's my story. The transfer case works. When I jack the vehicle up, the front axle(s) seems to work properly. I've pulled the battery, and the actuator sucks in, however it disengages when shut off. The 4x4 seems to work when under light loads, such as skidding around on ice (hard to verify, as its very difficult to skid around a corner while watching your front wheels. I'm guessing based on feel of the skid). However, when I pull up to a snow drift, the front wheels do not spin, just the rear. I'm suspecting a slight vaccum leak somewhere, causing the diff to only engage a little but let go under load...

Am I in the right line of thought here? Any suggestions where to look?

Do the HVAC controls do what they're supposed to do, all the time? That's a common symptom of a vacuum leak. When you shut off the vehicle (while in 4WD), I would expect the actuator to continue to be engaged for awhile, so it does sound like a vacuum issue. Whenever I have vacuum problems, I start under the hood with a MityVac and test each of the 3 vacuum lines that split off from the one-way check valve (HVAC, transfer case, and vacuum reservoir in the fender).

Well, I've had problems with the AC turning to full blast heat this past summer, but the vents seem to blow where they're supposed to.