Since getting the new tires the vibrations have lessened quite a bit. The 55 mph vibe seems to come and go. The new tires are a bit bigger and seem to have made the vibration start around 52 mph but again its not as bad. I could probably live with it if I have to. The higher speed one is still there. When I have time I'll try removing the rear driveshaft and going for a drive. Ive replaced most of the front end, axle u-joints, hubs, driveshaft u-joints, engine and tranny mount, tried different rims etc. I'm running out of things to replace. Still have to check pinion angle. With the new springs the back of the truck does sit a bit higher than it used to. Now I just need to find the time to do all of this.