1995 Ford Explorer probable causes

My 95 explorer is shaking and wobbling durring take off exceleration, and there is a loud humming noise while driving. I've jacked the car up on all fours and put it in gear with 4wd engaged, and it don't seem to do it but with the vehicle off I manualy turn the front wheels, with 4wd disegaged and the axles turn along with the wheels, idon't know if the front hub are sopposed to be free from the axles or not, and also it seems to quit the shake and wobble when coasting. How does this part of the 4wd portion actually work.