Does any one know of a solution to this auto problem?

I have owned several Chevrolet trucks (both pickup and suburbans). Without exception, the models with the 02 to 08 truck style (including the suburbans which didn't have a body style change until the most recent I found that somewhere around 75,000 to 100,000 miles, the front end would develop a knocking noise when traveling at low speeds over bumpy terrain. You can also feel the "knocking" sensation through the steering wheel. I even drove a friend's suburban once and it was doing the same thing. It just feels like something in the steering column is loose and knocking against something else. I took my first pickup with this problem in to a dealership and they said it needed a "front end lube kit" (best I can remember). Once it was installed the problem completely vanished. Problem is, on every other Chevy that developed the same problem, I have taken them in and no one seems to know what the problem is nor have they ever heard of a "lube kit". So does anyone know what I'm talking about and do you know what the solution is?