Yeah I would check the tires,and the u-joints,and the pinion to make sure the nut hasn't worked loose.
I'm betting the tires have a flat spot,or a bent belt,or just need balanced.

49 Chevy 3600
67 GMC 3/4 ton
71 GMC 1/2,daily driver
http://www.sofnet.com/~eaglefeather/

When I die I want to die in my sleep,not screaming in terror like his passengers