steering front shimmy.........?????

I have a 98 GMC w/350 4x4 when hitting above 55-60 mph the steering wheel wobbles/shimmys side to side....its anoying as heck!!!!!!
any thoughts as to why or a fix for it?? (my tires are in real good shape,pretty new, and all even tread) anyone ?? thanks in advance for any help!

It's a wheel balance problem. Maybe a weight flew off. Maybe a bent rim.

Have them re-balanced or rotate front to back and see if it goes away.