What would cause a 1993 Nissan 2wd to shake violently at speeds above 50 mph and drive like it is on rails below 50?

Check tyres (all) for bulges,tread separation and for egg shape.failing that have your wheels balanced at a garage or tyre specialists. if you are sure that the front end of your vehicle is sound and in good repair tires wheel bearings ty-rods etc., the check to see if you have any slop or movement in your carrier bearing, or u-joints. some movement is normal if you shake your drive shaft from noon to six but you should have no movement if you shake it from three to nine. hope this helps.