Chevrolet Repair: Chevrolet Silverado Shimmy, bad ball joints, kyb shocks

Question
Robert;

I have a 2001 Chevrolet Silverado V6 with 114K miles.

This truck has always had a shimmy (not in the steering wheel) at speeds right around 60 MPH.  The shimmy goes away at speeds outside of the 59-64MPH range.  I got a nail in my driver's rear sidewall, and had to replace one tire.  Rather that doing one tire, I did both rear tires.  I drove the truck on a 3 hour trip and the ride/handling was horrible-which i attributed to mismatched tires.  ONce I got home, i replaced the remaining two with the same brand/style.  The ride is still horrible.  I've taken it to a shop and they have balanced/rotate all 4 wheels several times with their Hunter RF balancer.  The ride is still awful.  They did say i have a slightly bent rim, and need new shocks.  I replaced the rim (which helped a little) and put 4 new KYB shocks on myself.  The ride is still the exact same, which is bad.  The vibration is a side to side movement that you feel in the seat and floor, not the steering wheel, and it only happens from 59-64 MPH.  I have not had it aligned in quite awhile, and don't want to throw more money at solutions that don't work, do you have any ideas?

Answer
Hi Ryan.... there could be a couple different things...check the front wheel brgs for play. check front end IE: bad ball joints....check to see if you have the exhaust touching or rubbing on anything, check motor mounts and trans mount...check the u joints and driveshaft, check lash in the pinion at the rear end...because you feel the vibration in the floor and not the steering i would look hard at any of the mounts...check that the rubber in the mounts is not broken or has too much movement in it causing it to vibrate against the crossmembers....also check the harmonic balancer on the crank....chasing this kind of problem can drive you nuts... i have even seen where the computer had broke free and was vibrating as you drove the truck....so don't overlook anything....hope this helps