Vibration Problem and Bed Shake

I own a 2004 Toyota Tacoma Prerunner V6 SR5 w/ TRD package. I bought this truck used and it came with P265/70/R16 tires on it. They were not the Rugged Trails that come with it from the factory. I have Runway Enduro HT's on them and they are apparently entry level tire. To make a long story short, I have a bed shake and vibration (in seat which indicates rear of vehicle) between the speeds of 50 - 60 mph. Toyota has looked over the truck twice and said tires. I went to get a 2nd opinion and the tires Road Force tested and the issue still persists.

During the test, one rim failed, but whas balanced and put on the RR. My issue is on the LR. I took it back to Toyota and they put different set of rims/tires on it and drove it. They said the problem went away. I came down and drove it and it seemed better, but on that part of the road they and I drove it, I realized after that my truck really doesn't do it their either.

Tires Plus also put 2 new tires on the back (Duelers HT) to see if it went away. Supposedly it didn't. They said it isn't the tires, but Toyota says it is. Of all the things I did, and I am probably forgetting some, my co-worker, who used to be an ASE Certified mechanic, before chaning careers said the following:

At one point the rim that failed the road-force test was likely on the left rear. It could have been made worse by being put there in combination with an out-of-round tire or it could have been improperly balanced. Now, a typical shock is only performs to 95%+ of it's intended performance for about 75,000 miles; however, overcoming a redundant vibration can wear out most shocks in less than 10,000 miles. It won't wear it out completely, just in a small range where the shock is constantly overworked. Within this range the shock becomes very "loose" and leaky (the fluid seeps around the plunger internally). Now, I know what you're thinking-- the Toyota guys checked the suspension-- but let me tell you what most mechanics' idea of checking the suspension is. I kid you not, I was taught in auto tech school to go to each corner of the car, push down on the bumper forcefully on each corner, let go, and watch how many times the suspension "undulates" before returning back to still. If it "undulates" more than 2.5-3 times, the shock needs replacing...that's it. That's it? Isn't there some sort of machine that you put the shock on which tests it dynamically? There are, and they're expensive, and most people don't know how to use them, and it's an old model or isn't calibrated right, you won't get the right test results. Not only that, but it tests over the full range and usually misses leaks in increments shorter than a half-inch. Most mechanics' "testing" of shocks is the equivalent of making a purchase to buy a car by kicking the tires. But I digress. Here's the reason I think the new tires didn't make the vibration go away completely: You're driver's rear shock is dead within a short range. At 50-60 mph "some roads" cause a resonance the starts a little "wheel hop" (roads have imperfections too). As the wheel hops up against the spring, the spring pushes it back towards the ground and this happens freely within a short range of travel.

What is everyone's thoughts about that statement? I don't know much about shocks other then the basics when they go bad. Is this logical or possible? Does anyone out there have or had the same issue with their Taco? Apparently Tacos seem to be a pain to balance. I really want to go buy Bridgestone Dueler AT Revos ($150 each) but am not confident that will solve the problem. I think the Tokico shocks on this truck run about $80-$90 for the rear. My co-worker thinks I need to replace the shock and the tires and then probably the rim since that rim is fine on the left with a tire. I am going crazy. Thanks for any input you have.

Ed

Have a tire shop check the mount and balance. A bad shock isn't going to cause a vibration. Things to look for are bad u-joints, a bad center carrier bearing on the rear drive shaft, a bad axle bearing, an out of balance/out of round tire, damaged rim, or a brake problem.

The push on the corner and count the bounces is a actually good test for shocks. All a shock does is dampen the movement of the spring. If it stops the movement in one or two rebounds then it is doing its job.

I have a hard time believing that a "bad spot" can develop in the shock. A shock is just a piston in an oil filled tube. As the shock moves the movement of the piston is resisted by the oil. The resistance provided by the oil is what dampens the movement of the suspension. There is no way for it to be bad in one spot. If it was bad it would be bad through the whole range of movement.