Chevrolet Repair: front end chevy, pick up 4x4, broken tie

Question
steve, i have a 1998 chevy 1500 pick-up 4x4 5.o L engine. vechile has 120,000 miles on it. last night out of the blue the front end started to shake ( shimmy ) when i hit 45-50 mph. thought i had a flat. no such luck. experimemnted again and at 45-50 mph same occurance ( the steering  wheel also shakes ? ). took hold of each front tire neither shakes or feels loose. put new tires on @ 80,000 miles. the outside of the tires are a little worn and i plan to rotate them today ( should have been done months ago! ) went under the truck and did not see any broken tie rod. any suggestions?  ball joints? the tires were balanced when they were put on and it does not APPEAR that any of the lead has fallen off              waiting on your reply  thanks   MIKE  

Answer
If the front end checks out ok and everything is tight, it has to be an issue with the tires, between 45-50 mph is a classic case of out of balance or a bent rim or a out of round tire, have them check again or rotate them to the rear and see if the problem goes away, if it does that will answer your question