It might be the cords separating in the tire as well.
Have someone slowly drive the truck and look at the tread on the tires and watch for abrupt changes in the pattern or a bald spot in the tread if you have had the shake for a while.
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews