I am confused as when this vibration happens??
Is it in first, or one of the other gears??
If it only in one or two gears, I would think it is a transmision problem. Though maybee it is a speed related problem, tires out of ballance, bad U Joints in the driveshaft.
Tell us about the truck you are looking at, and where you are from. Who know maybee someone here, might know of a better truck close to you.
Maybee you might want to pass on that truck, and find a better one.