I have this '86 3/4 ton chevy 2wd truck. After I replaced 2 very shot tires (broken belts), and got some other issues worked out, I now have a bad vibration starting around 45-50mph, and by 65mph it is usually so bad that the truck feels like it is "skating" on the road. But, sometimes it is worse than others. I have rotated the tires, no help, and tried dropping the truck in neutral at speed and that didn't get rid of the vibration either. It is coming from either the driveline, or the rear end. Short of unhooking the driveshaft and having someone tow me down the highway, is there any way to narrow this down one way or the other?