Stevebolt, Focusing on the statement of the "bouncing" being rythmic, not necessarily road surface related makes me think,like others said, the tires/wheels have out of round condition.
I have seen people fall into a trap allowing this condition when wheels were used that were not properly centered by the bore in their center which should closely fit a locating diameter on the axle or hub.
Watch this fit on after market wheels--the studs or lug bolts won't properly locate the wheels.
I would put the rear axle on jack stands or blocks with the wheels well clear of the ground-- crank the truck and make the wheels turn not very fast and look at the runout of wheel rim/tire od/wheel center hole.
I have seen bent axle cause what you are seeing.A bend in the axle shaft in the housing can make the pilot diameter and axle flange face for the wheel runout.
The condition could have been pre existing to your spring work but masked by percieved shock/spring problems.
Last edited by enginenut; 06/22/2012 1:46 PM.
