Vibration at ~1400 rpm

I have a 05 Prerunner, V6 with 170,000 miles. For the last ~25,000 miles I have experience a vibration that seems to occur somewhere in the front end of the vehicle. It always occurs during normal acceleration, never during aggressive acceleration. It happens at around 1400 rpms. The frequency of the vibration changes with the gear the vehicle is in (auto trans.). In the lower gears it is felt thru out the vehicle at the higher gears it feels like rumble strips on the road. I just realized that the problem seems to be a hesitation problem that is associated with the transmission. If I take off in Drive it vibrates, it I take off with the shift stick in 4th no vibration. All suggestions are greatly appreciated. Thanks in advance.

 

we're you able to figure out whats causing the vibration? I notice the same thing on my 2006 Tacoma V6 SR5. I'm still trying to figure it out

 

Change transmission fluid and remove driveshaft and check u joints