1996 Chevy Tahoe Transfer Case?

We experienced a problem after having a new motor put in this vehicle 3 yrs or so ago. When the vehicle was in reverse, it makes a slamming noise { like you ran into a wall}, then a loud whining noise. This did not happen all the time, but enough to worry and annoy us. We took it to Aamco Tramission, the same place that put in the new motor, and we were told the transmission needed to be rebuilt. We had the rebuild done and bought a life time warranty.
We continued to have the same problem and each time we took it back to them. We have been told they could not duplicate the problem, they greased the transfer case, that we needed a new transfer case and numerous other things that we paid for. We are now having the same long whinig noise noise in drive and we have to put in neutral and pray it stops so we don.T have to stop in the middle of the road
We decided we could not trust them so we took the suv to another Aamco shop and we were told, without out them taking anything out to check it, that the problem was a part that had to be put on as one of the first parts when the transmission is rebuilt. They did roll their eyes when we told them where we had the rebuild done. They could not do the work there since the lifetime warranty was bought thru the other location, he did say they could pay them to do it and they would complete the work.
This did not seem to me that they would be lying because it knocked them out of a repai.
We took it back to the original shop and it took them almost a week to call us and let us know it is the transfer case not the transmission and it will cost $420.50 to pull out the transfer case and when they open it and find the problem, the will apply that to the cost of the repair.
The shop manager did call the insurance company about the lifetime warranty and I think they would have to repair a transmission problem and not charge us. We have paid these people over $5000.00 and I just can.T see paying them anything else. What are your thoughts on this?
Can they know it is the transfer case without opening it and checking it out?
Is this what a transfer case problem sounds like to you or is it more apt to be a transmission problem?
Please help steer us in the right direction?
Thanks, Linda