Ignoramus3720 fired this volley in
:

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

In addition to the suggestions about U-joints and trannies (but not
tires), It could be a torque/speed problem related to the engine
performance. It might be missing at a certain torque and speed AND
vehicle speed range.

I lean more toward it's being a tranny problem.

When's the last time you completely drained, flushed, and re-filled the
transmission?

Lloyd