I have a 79 K5 w/ 4" lift and 35s. At about 40MPH on up it begins to vibrate the truck, in a pulsing pattern. It runs smooth, the grows into an annoying vibration then tappers back down over the corse of about 10 secs. It does this over and over. I has done this since I bought it about a year ago. I just put a new crate motor in it. I figured it was tire related, so I put brand new rims and tires, still no help, I had the drive shaft legthened to the proper size for a 4" lift (this was never done) and had the drive shaft rebalanced. THis helped a little, but I still have the same problem? I have locking hubs, so the front drive train does now move at all while driving. What else could it be. could the rear axle shafts be bent? or un blalaced?