My brother has a 2006 Ranger 4x4 and it just recently started shaking really bad side to side whenever the 4WD is engaged. Along with the truck itself, the steering wheel shakes side to side and the truck can't be driven over 20 mph without feeling like you will completely lose control. Like I said, it only does it when in 4 wheel drive, and then when you switch back to 2WD, you can both hear and feel a really loud CLUNK and the problem is gone. Any thoughts????

 

Check the front drive shaft cv.