98 yukon 4WD problem

I have a 1998 Yukon w/electronic (automatic transfer case).The transfer case seems to be locked in auto 4WD even though the indicator light says it is in 2HI.The front driveshaft is spinning when transmission is engaged, but the front wheels are not turning.I am getting a bad vibration from the front driveshaft turning.

Any ideas on how to fix?