I went thru hell with my 01 Tahoe HVAC. Before doing any mechanical work, try this: Remove the radio fuse and the heater fuse for at least
10 minutes. The fuses are behind the panel in the left end of the dash. This will cause the unit to re-initialize and find its stop limits. It's worth a try, and yes the dealer doesn't have a clue about this.
If Mr. Goodwrench knew how to use his equipment, he could diagnose this problem, but that ain't gonna happen. I had my Tahoe tied up in warrenty work for months. I just would not accept the car till it worked right. I had to force the issue thru the BBB arbitration.
Derek is probably right, but you should try anything before tackling this nasty job.