Mike, how about the differential in your truck, is it in good condition? The conditions you are describing ring a bell to me, especially the vibrations not occuring under load, just cruise and coast. Sometimes, uneven wear patterns on the ring and pinion gears in the differential can cause the condition you described. I seem to remember reading an article in "Classic Trucks" magazine which gave a list of symptoms and causes within the differential. This list was a sidebar to an article on differential and rear suspension overhaul on a early 70's Suburban. I can't remember which issue it was in, I'll have to check my "library" and get back to you with another post a little later.
Wannabe
no truck right now...
one day WILL HAVE an Advance Design Chevy...
