transmission for 01'

My Yukon is driving just fine and then goes into limp mode 2nd gear and won't shift. The mechanics are baffled as the transmission is good, their is no damage to anything inside, and when doing the maintanence, the pan was clean with everything looking clean and good. No codes are registered and the engine light does not come on to indicate a problem.
Anyone else have a problem as this and what was the cause and how did you find it and how did you fix it. The shop has called other dealers, and no one has heard of this happening. Thank you for any help you offer.

Hi,
please add more detailed info on the sequence of events, and what year, model, engine, tranny.
I understand that the tranny guys already checked things like the Shift Valves, Solenoids, Clutch Components and the Quad Driver Module.
Take care, Eggert :bananasmi

since this was posted, I have talked to the shop and gotten more info. The tranny maintanence was done and everything looked great. He was able to make by pass 2nd and go into 3rd and so on. All his dianogstic testing found all in order and the computer showed it engaging, but mechanically no going past 2nd. I talked to the GM Svc center who gave me the number for dealer here in Anchorage, and the shop forman suggested that because of the remote, it may be the ingnition as this is what he has seen to cause this type of problem. The other shop he would agree, but it will engage. They are looking to replace the tranny with a reconditioned- 3yr warranty and 50,000 miles. We have had the car for 2.5 years and it has 60,000 miles on it. It came with the remote and we have had no problems until now. Some else suggested a shift solenoid, however, the shop has said that they were good. In the meantime, the cost is adding up.
I greatly appreciate any help, thank you

I had somewhat the same problem. The problem began at about 40k. After I put it into drive the engine would rev before the tranny engaged then it would clunk into first. The subsequent gears would be sluggish and once in a while the shifting would take on the same characterstics in the upper gears as it had from park to first. Well, my 01 Yukon xl had 50k and I needed tires because I had those horrific Wilderness LE's on it and it also needed it's first tranny fluid change and guess what? No more problem and the car drives better than it ever has. FYI, the tranny was clean with no mechanical problems either.