Mystery vibration at 25 mph can't be diagnosed. It's not the front end!

1) It has yet to be verified whether this is an engine speed shudder, a driveline speed shudder, or even a wheel speed shudder. Since We here on the forum have almost
no way of experiencing this 1st hand, slantsixdan gave the OP links to literature which should help Him figure this out.
2)If it is engine speed it wouldn't be "only at 25", and engine load/strain increases in higher gears, but the OP needs to verify this. I have already stated how.
3)If it is driveline related it will only increase with vehicle speed, regardless of the eng. RPM...the only exception being the torque converter/input shaft-drum assy.
4)It is very common for a tire with a failing belt to tug or deform at precisely the 20-25mph range, on the front it tugs on the steering wheel, and in fact just happened
to Me on My 'rolla. I was hoping that set would last Me 'till I mounted the snows, but nope, one day moving out.....tug tug tug..... so it's not impossible for a belt to
show it's given up the ghost under torque by causing the car to shudder.
5)Frequency frequency frequency, what is it in relation to the above systems................................