I have a 2007 Hummer H3, driving in 4 wheel high it quit pulling had to put it

I took it to a dealer and who test drove it. It didn't at up and the scan only showed a themostat code which wold not cause this problem. It did it again on the way home and will know only go in high lock.

Same thing seems to be wrong with mine, even the sev temp light. Backing up one day I thought it popped out of 4x4 lock but the next sharp turn I knew it didn't. Then it popped and thought something went out going forward. Next day it pulled to the left like mad, thought I had a flat tire(since me low indcater light is always on)stopped to look but nothing to see. How do I get this thing out of 4x4 lock?