Slop in the ball joints can allow both caster and camber changes, which can cause a shimmy. I'm not so sure a driveshaft u-joint will cause it, but it's not good regardless. Check to see how tight the rod ends are on the steering linkages and the track bar. An easy way to check is to have someone move the wheel back and forth without turning the tires too much. Any 4x4 Dodge owner knows all too well about the slack-on-center in the steering. Obviously, you need to lie on the ground in front of the truck, and it needs to be running, so do this with someone you trust. Make sure the steering damper isn't weak or blown, too.