I've been having similar issues with my 2006 Yukon. I had originally replaced the Encoder Motor but that didn't solve my problem. I then found a post, that I can't seem to find anymore, that said this problem can also be caused by a loose connection on the cable to the Front Differential. I moved the pins on this plug just slightly so that they would make better contact put the plug back in and wala the problem went away. Well for about a year. Then problem started occurring again. My mechanic read the codes and said it was the Encoder Motor, I don't remember what the code he gave me was. C0327 I think. Anyway, I told him what I had found the time before so he dissembled the plug and cleaned it up but that did not solve the problem this time. I called OnStar to have the read the codes and they say there are not any codes in the system. I'm going to have my mechanic read the codes again and see what he says. Any thoughts? Has anybody else had problems where OnStar couldn't read the codes?