I have a 2002 Chevy Tahoe with a 4wd issue. I push the button for 4 Hi and the light comes on. The actuator moves in and out like its supposed to as well. I had it on a lift, put it in 4 HI, put it in drive and the front wheels turn. The problem is that I can grab the front wheels and stop them from rotating. Its like there is very little power turning the front drive shaft. What causes this and what should I look into?