Denny,
I had a simular vibration problem as to what you described in my 1948 3/4ton. It ended up being the slip joint in the driveshaft it had some wear in it and an odd vibration would start around 45 mph and was really noticable at 60mph. I coud tell it was not from the tires. Anyway I had the slip joint replaced and ballanced the driveshaft and that completely took care of the problem.
