vibrations in 92 f250

my 92 f250 has been vibrating between approx. 60 and 70 mph. its not like a steady vibration but more like pulsing vibrations, coming and going slowly and steadily. when i have weight in my truck it seems to almost stop but i can still notice it. i recently had an alignment and balance which didnt help the problem, as far as i know the bearings were also supposed to be repacked. anyone with any idea what else i should be checking to be causing this?? some help would be very much appreciated.
thanks