I have a problem that I noticed with my pick-up truck.

The problem is that at certain speeds, such as 40-45 MPH, I feel
vibration if I step on the gas pedal and try to accelerate. If I let
go of the gas pedal, vibration stops even as the truck continues to
drive forward. It has gotten worse lately. I have thought a lot on
what could cause it. It does not seem to be related to the engine
RPM. I watch the RPM vary and the vibration does not vary with it.

What could it be?

i