Why does steering wheel shimmy from 48 to 55 mph and also jumps up and down frequently?

Out of balance tires, or tread wear, or you hit something that has miss aligned your front steering system.

your wheels more than likely needs to be balanced one or more are out of round if you had the tires rotated before the alignment this would make the vibrating more noticible.but balancing should solve your problem.

Front tires may need balanced. Have the alignment checked. Try rotating the tires from front to rear.

Hello. Most likely you have an out of balance or out of round tire. A simple way to check tire balance - if the shimmy is present at one speed and not present at another speed, then you most likely have a tire out of balance [sometimes the tire balancing weight[s] can fall off. An out of round tire or bent rim usually shimmies at all speeds and replacement of the tire or the wheel is the only fix.
Begin by driving to a reputable tire store and have them check your tires, a free service in most instances. IF it is not a tire, it could be that you need a front end aligmnent.
Regards, Joe
PS please rate my answer. Thank you and happy holidays.

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.

If it shimmies with the brakes applied, it's a rotor problem....but like you say, it shimmies just at a certain speed with no brakes applied, check tires for out-of-roundness. If they look ok, you are looking at a balance problem with the tires.

Since you can make it better by unloading the tire a little I would suspect a wheel bearing. You also may just have a bad ball joint. Also if the tires were just rotated and you developed the problem you may have a defective tire.

I had a 2008 Supercrew that had that prob. One of the tires was defective. It balanced ok but at speed would vibrate. The dealer sent me to Discount Tire. They have a machine that will detect problems like yours. I don't remember the name of the machine. Have your dealer take it or send you there. I would think Goodyear would have the same equipment but maybe not. Good luck!

Most shimmies come from tire/wheel/balance combos. Before your dollars go down on 4 tires(assuming you have good tread), rotate tires & if there is a change you can figure out the offending unit

Tires, unless it happens when the brakes are applied, that would be warped brake rotors.