Shake

[rtm038]

Hello all,
I bought a 2006 crew cab short bed 4x4 with 82K miles last June. When I drove it, everything was great. The only thing it needed was tires and shocks. I had the local dealer replace the tires and shocks, plus take out the front leveling kit (spacer type) and rear Firestone Air Bags because I wasn’t running larger tires and didn’t plan on towing anything more than my 3,000 pound tractor from time to time. Well, after getting the truck back, I noticed that when the truck was in overdrive and running between 40 and 60 MPH, I could feel a slight shake from the rear, kind of like a tire that was out of balance. So I took the truck back to the dealer where they checked the balancing on all 4 tires and found them to be fine. Then they took out a carrier bearing shim, which fixed the vibration I was getting from a stop, but not the shake at 40 to 60 MPH. Fast forward a few weeks, I noticed a really bad shake, almost like having a flat spot on a tire. This was found to be a parking brake shoe that had wedged itself within the rear drum and was getting whacked by the backside of the wheel studs. Well, I had the brakes (rotors, pads, the works) replaced front and rear and that solved that issue, however the shake at 40 to 60 MPH is still there. So, my question is what could be causing this light shake? I’ve noticed that if I’m going down the road between 40 and 60 MPH in overdrive that the problem pretty much goes away when I activate the Tow/Haul feature. Is this normal for these trucks? My last 2006, which was also a crew cab short bed 4x4, never behaved this way.

Thanks
Ryan