When you push the 4 hi or 4 lo button on the dash I can hear a thumping noise from the passenger side kick panel. It will thump a few seconds then go right back to 2 wd. I got under the blazer today and had dad push the button, the motor on the transfer sounds like it is trying to engage and I tried turning the front drive shaft. The shaft will engage I between thumps and then free turn I between. Any ideas?