go back by discount tire and ask them to do a vibration check. have them show you any bent wheels. if they show you one that is really obviously bent that could be the problem. if its barely noticeable it probably isnt the rim.
have them check to see if the tires are out of round. if you bought a set of used tires i would think this would be the culprit. a lot of time tires will not be round and will result in a "hop" in the tire. while it is spinning on the balancer look at the surface of the tire. does it move up and down or side to side? side to side isnt a biggie, but if it moves up and down a good amount thats your problem.
if one of them is particulary out of round politely ask them to do a 180 to the tire. this involves breaking the beads and rotating the tire on the rim before reseating the bead. sometimes this can help.
if all the tires are out of round and rotating the tires on the rim doesnt help the only thing that will solve this is new tires.
another thing to check is for cupping out of the tires. run the palm of your hand along the tread of the tires, paying special attention to the inside and outside edge. is the tire flat and round? or does each individual lug seem to be ramped or have section of uneven tread wear. cupping can be caused by lack of rotation or suspension problems.
put the worst assembly on the right rear (furthest away from driver) and the best one on left front.
go out and talk to the techs working on your truck. this way you get all the information and make sure it is done right.
there will be no charge for this service so make sure to give the tech working on your truck a fat tip