Substantial front end shimmy at 50-55 MPH. All new tires, front end parts look good, ball joints fine.

Could be several things, including a bad tire or bent rim.
A shimmy usually starts after you hit a bump, which would point to a loose tie rod end or ball joint instead of the tire/rim.

Wheel alignment is out of specs. Loose front end parts

COULD BE BAD WHEEL BEARING.WHEEL OUT OF BALANCE OR HAVE A BAD PLACE ON IT.COULD HAVE WORN TIE ROD END.WORN BALL JOINTS WILL CAUSE WHEEL WOBBLE OR SHIMMY.

Check your ball joints. also, if the tires are somewhat worn many times they won't have enough rotating mass to overcome side harmonics and can shake but most oftern the joints are worn. No play is acceptable there.

rise the truck front end suspension, at the front tire gently shake the tire side to side u will fill play in that area. if there is play most likely its the inner our outer tie rods. you can also check the ball joints by moving the wheein the north and south position gently checking for play in the upp & low ball joints.

a few things tires out of balance-bad tire-rim bent-shock/strut bad-worn or loose front end component-ball joints-bearing/hub---if bad enough to shake/shimmy u should be able to see or feel problem under front end jack up car and with wheel on move side to side than up and down than in and out when u do this if u hear a clunk or see a part move that shouldnt [tie rod connection/ball joint etc] than u found problem

If your tires were not properly balanced then it will cause that. Also, if your rotors are warped then icould cause this to happen. YOu would also notice it when braking if rotors are warped.

have u had your shokes check

not a big problem. It may be tie rod wearing. If you hear a click when turning it would be the ball joints. If it does not make clicking sound, then most likely tie rod ends or improper alignment

It is possible that one of your new tires has what is called a slipped belt which is basically a faulty tire.

If they were able to successfully perform the front end alignment you can almost be sure that all of your ball joints, tie rod ends etc are ok as these really don't attribute to "shimmying" felt in the steering wheel.

The are several common causes of shaking felt in the steering wheel:
1. Out of balance wheel/tire combo
2. Slipped belt in a tire
3. Bent wheel

Your shop should be able to pinpoint the cause easily.

Take your car back to the shop that replaced your tires and tell them what you are experiencing. They can check your new tires again to make sure the belt on them did not slip.