Hi everyone,
I went through this a while ago with our 88 Comanche. At about 45 mph or higher, when hitting a bump on the road, the steering wheel, front end, and entire truck would shake violently. All I could do was stay off the gas and brake and wait until the speed dropped to below 30 mph.
Front end was tight. Alignment was good. I had factory alloy wheels, and had them rebalanced with stick on weights, which helped a little. Replaced the steering damper with a heavier model, which helped a little. But there was still a significant "death wobble".
Problem was not solved until I got a set of junkyard steel wheels, used the same tires on the steel wheels, and had them balanced with real weights. Haven't had a problem since.
Gene