I kind of have the same think but mine is driving down the inter state or any where realy I get a on of off vib that last about 2 sec if i step on the gas or hit a hill it leaves I rebuilt the front and rear shafts about 3yrs ago i notice mine when I put a set of helper springs on I dont know what that would have to do with it I didnt have to take any thing apart to do it . I have sence taken them off and replaced the rear springs with some from Quardtrack but still have the vib have had tires balance 4 or 5 times in the last 2 yrs went thru every thing in the front end still cant find it .