@GRoy
"Taking it to the dealer was a waste of time because all they wanted to do was reprogram the controller."
GRoy, did they reprogram or they just wanted to reprogram?
As far back as March, 2007 GM found problems with programming in the Assist Step Controller and issued a TSB (Technical Service Bulletin) #07-08-61-004 to help mechanics diagnose complaints like yours, involving 2007 Yukon Denali models and other 2007 vehicles. The boards were found to be intermittent or stopping in mid-travel. Some are found to have set DTCs (diagnostic trouble codes) B051C22 or B051D22.
This could apply to your vehicle.
Bulletin Link:
CSA