Problem with transmission kick down feature etc Help

Hi. Brought my 01 GMC Yukon XL in to be serviced. Approx 27000 mi. The SES light is on. On the highway it doesn't go into overdrive and the kickdown feature when hitting the trottle isn't working. According to the computer whatever happened, happened 400 mi ago and while it was standing still. The mystery deepens when they say they think it is related to non GM wiring? They thought I installed a remote starter. I never had anything done to the truck since getting it in 01?? There is a small black box to the right of the rearview mirror on the windshield that nobody (dealer mechanic) knows what it is. Their claiming it was not a factory installed item that year and they though I had a remote starter installed that may be contributing to this problem(s). Now I'm really wondering what this is all about? Any ideas?

If you didn't buy it new, think I would be digging into the little black box and finding out what that is, if nobody at the dealer or the mechanics or otherwise recognize it, I would be concerned about what it could be ....GPS for some kind of theft protection maybe ?? If its really a remote starter it should be easy to figure that out, follow the wiring.

There are some GM tech bulletins for trans problems on those models relating to no 4th gear and the SES light being on...
As far as the blck box, could have been for a car phone or some kind of gps theft tracker.

I had a similar problem, turned out to be a clogged fuel filter.