I have a 1999 Blazer 4x4 that shakes at 75+ mph. It's not a constant shaking and doesn't seem to have anything to do with the type of road, asphalt or concrete. The whole car will start to shake for about 1/4 mile and then quit, a few miles later start shaking and then quit. Below 75 mph it's fine.
Today I was changing the brake pads and noticed some green grease coming out of an axle boot on the front drivers side. It's slung around onto everything in a circular pattern. From what I can tell the boot looks ok, not ripped or cracked. Could there be a problem with a seal, bearing, ??, inside this boot causing the shaking.