After offroading with friends yesterday, we were racing home and when they were behind me, told me on the FRS radio that the left side of my bumper cover was lower than on the right side. I thought that they were joking until we returned home. Sure enough, it was. You could fit two fingers between the top plastic piece and the bottom plastic piece on the passenger side, but three fingers on the driver's side.
I scheduled my Avalanche into the service department of my dealership today and they thought it would take one hour. Over four hours later, my bumper is now level again. My problem may have been more serious that yours, Bear. It seemed to be more than just the bumper cover in my case. The did manage to adjust it though, but it was very difficult. They loosened off all bolts connecting the bumper to the vehicle, and then released air from the rear tires to get the truck to squat more. Once that was completed, they put a jack under the driver's side end of the bumper and started bringing it up, easing the vehicle's weight onto it. Various things got in the way, and it took them over four hours until they finally got the bumper up into place where it should be and then tightened it down. All of this was covered under warranty.
If your dealership claims that it is not repairable, I would ask for a second opinion. My dealership told me that there is not much room for adjustment, but there was enough to repair the problem in my case. At the very least, it should minimize your problem.
I was going to take pictures, but by the time I thought of it, I was at the dealership already.
I hope that this helps.