2000 Bravada Transfer Case?

I have a 2000 bravada with smart trak, and recently it's developed a heavy lurch accompanied by a loud clunk after 5 or so miles of driving, once everything is warmed up. trans. is definitely not slipping, as I can still squeal my tires on takeoff. doesn't seem to happen in reverse, but then I don't back up very fast. Seems that the hotter it gets, the worse and more often it happens. Nearest we can figure is transfer case going bad? any ideas?

Have you changed the transfer case fluid regularly? The GM dealer has the special fluid for it. If it hasn't been changed regularly, you may need to change it, drive for 100 miles or so and change it again.

I've only owned it for a year or so, but the previous owner had the entire transmission replaced by the dealership in 2007, so I would have thought they'd install the correct fluid then. Guess I'll have to double check that