rear vibration

[Locutus]

I have a severe vibration that begins about 70 mph, I suspect tires but it seems to severe for that, could the fact that I put heavy duty springs in the rear be intensifying the effect of an out of balance tire. I have not had the truck very long and have not had many oppurtunities to drive it at that speed so I don't know if this is new or not. Recent work done involved putting in new springs, shocks, and I had the rear gears professionally changed from 4.11 to 3.08(fuel mileage and this is to be my daily drive plus I want to haul my race car to and from the track with this truck hence the heavy duty springs. ) I am running a trubo 350 that is not orginal. It was orginally a 3 on the tree( I wish it was still a manual) Any ideas on locating this problem.

Mike

PS I hope its not the rear end cause they had my truck for 2 months installing those damn gears.