Chev Tracker bangs in and out of 4WD

[pafoxhunter]

Background: 2000 Chev Tracker 2 Dr 4WD, about 80k miles, very low usage of 4WD. Shifter slides easily in and out of 4WD. Sometimes a little slow to engage, probably normal, but disengages reluctantly.
When engaged there is an occassional loud, hard, bump or thump that can be felt in the drive train. It usually thumps like this when it disengages too. Mechanically, a quick look seems to show that the front hubs are always engaged, and that the front end is engaged by some sort of pneumatic pump and cylinder. I can't tell whats wrong here and need someone to point me in the right direction. Thanks.