1. Check tires for out of balance
2. Check CV shafts, if they are going bad, will act like that.
3. Can't remember what 3 was.
My truck did the same thing, and my buddy's did the same also. Mine is a 97, buddy's was same year I think CC dually. His CVs went, and at 40 or so mph, they should prett bad. Mine did it too, so I put new CVs in, still did it. Put my winter tires on, and it went away. Mounted up new tires, still wasn't there. So if tires are way out of whack, it will cause DW.