Hi there, I am fanatic about my vehicle.
I am noticing upon acceleration i can "feel" the shifting around 30-35mph, im nearly the first owner and do not recall ever noticing this before. If i accelerate FAST its not as noticable, but i drive slow, and its FELT. its not  "clunk, not a knock, not metalic, its only a felt "thud" as it shifts into the next gear at that MPH. upon hitting 50mph the final gear is not felt, only heard.
it is not hard or crunchy, but like a slap through a thick pillow.

I have to say it think its harder of a thud when its very cold. As if someone bumped into the rear of the truck very gently.

any ideas, why one gear be felt, and the others are seamlessly smooth?