379 pete vibrations

Not sure where to start on this but my 379 pete will vibrate under acceleration and cruise speeds in 9th and 10th gear. Sometimes on flat ground but mainly going up hill. Does it less when unloaded more so loaded. Truck has low air leaf 10 speed od trans. Where should i start first to look for issues. Has new motor mounts and front shocks. I have new rear shocks just haven't had time to replace them as of yet. Its also internment on when it does it, no rhyme or reason when it does it either.

 

front tires are brand new along with rears. I dont feel anything in the steering wheel it comes from the floor.

 

Its had done it since ive owned the truck over the last 4 months. Thought it was motor mounts replaced those and it got a little bit better but still does it. Some days it will be very minimal others way more noticeable.