Wheels out of balance.
Cracked wheel.
Did you say you rotated them and it still does this?
My first thought is, if it gets worse w/ speed, it is a balancing problem.
tie rod ends good?
SgtDel
aka "Andy"
www.delbridge.net
1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4