Truck Pulls to the left at times and i feel a slight vibe in the seat...ideas?

Besides the alignment and balancing.. It's possible what your experience is caused by an out of round tire. We have gotten several people with this problem. We usually rebalance the tires after we switch the location of the tires.. for example the LR would be moved to the RR and the RR to the LR.. assuming you don't need a rotation. The vibration in the seat is usually caused by an improper balance on one of the rear tires and the pull is typically due to an out of round tire.. so If they switch the locations and it pulls to the opposite side then you know it's one of those two tires. Sometimes the problem is eliminated when you do this. If it pulls the other direction urge the installer to replace both.. I would def watch them when they balance the tires. It's possible they don't even rebalance the rears... A lot of places will skip those since it's less likely you will feel anything. If it's an out of round tire.. it could cause both the pull and vibration your experiencing. Have you had the tires rotated since install yet? If so the vibration would be more noticable in the steering wheel and a lot more noticable. Just my 2 cents.. Best of luck!