Yukons/chevys have a tranny problem. Check engine light will come on pulling code p1870, which is for the tranny.
If you turn your car off for a minute, it will reset and wont happen till you drive again for a while. Mosly a hard 1-2 shift.
Most places will tell you that you must replace or rebuild the tranny. Not so. All you need to do is replace the tranny valve body, and MAYBE the solenoids. That will more than likely fix the problem at a fraction of the cost.
Personally, it doesnt bother me. Been driving for 2 years with the hard shift, of course I only get it once in a while cause I dont drive so much.
Simple google seacrh will give you a plethora of details:
http://www.google.com/search?num=30&...n+transmission