I have a 2003 GMC Envoy 4X4 that is having issues with the 4WD engaging (it does'nt). I have installed a new TCCM unit and a new transfer case encoder motor. I still do not have 4WD engagement. When I had it on jackstands (driver side both wheels off ground) changing out the the TC motor I noted that the rear wheel rotated in gear in 2WD and both front and rear wheel rotated in 4WD. But when all four wheels on ground the front axle is not getting drive power in 4WD. What I have not done and am thinking about is having the electronics flashed although the TCCM was a used unit. Also the transfer case lube is coming up on 50000 miles. Any thoughts?