You're describing pretty classic symptoms of a driveline vibration. Try this- - - -Find a screw-type hose clamp big enough to go around the driveshaft and secure a 1-ounce wheel weight to one end of the driveshaft, and mark the position of the weight with a sharpie, chalk, or a tire-marking crayon, etc. Test drive, noting the severity and speed of the vibration. Move the weight around the driveshaft in 1-inch increments until the vibration goes away or is at a minimum, then have someone mig-weld a flat washer to the shaft approximately the same weight and position as the test weight. Problem solved!
Jerry

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