Awd problem

Hi all, I'm Larry and I'm brand new here. I am buying a '97 Bravada with a bad shake in the drive train. My neighbor is a retired Olds mechanic and he drove it about a mile and said he is almost certain that the Smart Trak transfer case is malfunctioning. The vibration gets better at speed, almost disappears in turns, is not nearly as bad for a minute if you let the car sit and idle for a while. There is a chirping sound sometimes. My regular mechanic, a normally very competent guy, and my cousin put a new left front axle in today. The axle helped nothing. My neighbor said to change the transfer case fluid and replace it with the "blue" GM fluid for the smart trak system but the Olds dealer said the "97 doesn't take the blue fluid. It is a Borg-Warner case. We changed the fluid and put in Dexron lll. We are spinning our wheels here and need some advice. I am getting a very nice car at a great price so I will spend what it takes to fix it, I just need to know what to fix. I will post this on the other GM board as suggested below. Can anyone tell me where to go next here? Is there some where to buy the parts cheap? Can the guy who rebuilds our trannies rebuild this? (I will call him tomorrow.) Thanks in advance and Happy July 4 to all of you.

i had a 91 that did that lol it's very known for awd to do it because all wheels spin at the same time it could one of three things////..... try this first (cheaper) replace u joints all four two fronts and two rears/ if that doesn't work take front drive shaft off and drive it (don't worrie you wont blow anything) i put a 350 sb in mine and had to what three weeks for my front drive shaft to come in so i took it out and drove it rwd for a while/ then take rear off and put front back on find the vibrating/ third thing is check all mounts both motor and one tranny mount that can cause a vib and a chirp from not staying still