I investigated this further. The drive shaft (prop shaft) does not
appear to be loose.

The vibration seems to be related to a gear. If I drive wnd watch RPM,
at around 35 MPH, the vibration starts as soon as the RPM drops,
indicating a gear change. Again, it only appears when I push on gas,
and seems to be further limited to that gear.

i

On 2013-10-08, Ignoramus3720 wrote:
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