I also had a floor vibration about 52mph that I fixed last year. This is the process that the driveline repair shop had me do to find the cause. If you are uncomfortable then Les Schwab will be glad to do it. Test each following conditions at the vibration speed: 1. Rear axle on jack stands. 2. In gear and note speed at vibration. 3. Unbolt rear axle flanges pull axles out to disengage from diff. 4. Remove rear drive shaft. 5. Remove center drive shaft. 6. Remove forward drive shaft. Stop. If you still have a vibration it could be the park brake assembly damaged or engine has problems. My problem was when the frame was extended the forward driveshaft was improperly lengthened and was out of round.
