I had the problem a couple years ago, couldn't figure it out then either. It was time to replace tires so after that it only did it 1 time afterwards. After a couple years, the problem is back. Of course I can't reproduce the problem when someone is with me to be able to feel it but here's what happens. It's happened at speeds ranging from 40-70 so far. It starts violently shaking and I literally have to get the truck off the road and come to a complete stop. It's happened on the interstate, on bumpy back roads, anywhere, it's not picky. It doesn't matter the temperature or weather outside, doesn't matter if the truck is already warmed up or not yet driven for the day. I've had the tires rotated & balanced, problem still there. Supposedly, the u-joints are ok too. When it starts shaking, I've taken it out of gear, ran down through the gears, sped up, still does it. Only way to make it quit is to come to a complete stop, can't work through it by driving it. Sometimes it will start acting up a mile down the road, other times it's ok for a few days. Any suggestions? It goes to my mechanic in a couple days to see if they can find it but it's hard to explain when nobody else has felt it happen.

EDIT - Sorry, it's a 99 XLT, 4.2, manual trans, 75k miles

 

Have them look at the emergency brake system. I think it has to do with a part failing. Let us know what they find.

 

Another thing to look at is the motot mounts and transmission mounts. If they have loosened it could cause this by throwing everything off-balance. I have a similar problem on my 2002 SuperCrew 4x4 but not as severe. I believe the vibration in mine is from something loose or rubbing in the exhaust system (Flowmaster dual cat-back.) I have had the exhaust on for a year and the vibration just started recently.

 

You don't say what kind of truck it is, but does it have a straight axle? My mom had a '93 Jeep Grand Cherokee, and it did the exact same thing. On the Jeeps they called it a death wobble. Her's was repaired by replacing the steering damper, loose suspension components, and getting a wheel alignment. It never came back over the following 5 years she owned it.

 

I managed to get it to act up last night going home but of course not today when it went to the garage. But they did find that the expander cam on the e-brake was froze up on one side. I had found another post yesterday about it that described exactly what mine was doing. Seems that it is the typical e-brake problem that everyone has had!