00 Tahoe- 4wd not engaging AGAIN

My 2000 Tahoe would not shift into 4wd last winter. My GM dealer replaced the transfer case encoder motor and module for $1000. This winter again it did the same thing- if you push the buttons for Auto 4wd or 4hi, the button light will change, but the 4wd is not actually working- no traction at all. It will not shift into 4lo at all, the light beeps for a while, then goes back to previous position. I brought it back to the dealer and they say they think it is an altogether different problem, transfer case is probably shot because the fluid is black and there are probably some bands or something worn out. Is this a load of crap or what? At first they told me the 4wd worked fine, I said no, it doesn't engage. They checked again and said it was fine. I said no, it absolutely does not work. Then, lo and behold, when they checked it again suddendly it did not work at all. They thought they would have to pull the transfer case to see what was wrong, and repair may run to $2000. I'm a little reluctant to say go ahead. How do you know if the problem is in the transfer case, or if it is the actuator motor. Doesn't the actuator cause the actual shifting? I also am curious about a service bulletin that GMMerlin mentioned in a thread about the GM transfer case, bulletin #02-04-21-005A (slips in 4wd, noise, vibration). Is there a common problem with these transfer cases? Does a service bulletin imply that there is a problem that GM dealers will repair at no charge, or is it just the technical report on how to repair a problem? It refers to yoke engaging, and front and rear wheels turning at same rate. I told the dealer that since new this vehicle has always jumped and grabbed a little in the front wheels when turning sharply at slow speed in 2 wheel drive. Is this normal? He said it should not do that.
Any help or comments would be apprecaited.
Thanks

Transfer case went out in my Tahoe at 32,000. I was lucky it was under waranty. I could tell something wasn't right for a long time. I took it in several times and they kept telling me that everything was normal. Finally it started spewing fluid. Then check this out.... at 39,000 the carrier bearings in the front differential went out. Had to have the front differential rebuilt along with a new steering box. The steering went almost completely out. My truck was doing the same stuff you were talking about. Especially in tight turns, the front would start jumping. I'm getting rid of it. I don't want these headaches now that the waranty is out.