Maybe it is on the transmission. Is the fluid clean and topped off?

 

Fluid isn't perfect...I have actually wanted to change it before but was told that it's "lifetime" and shouldn't be changed. I balked at this, but have had that repeated to me more than once. I'm sure that sounds ridiculous, but even the local stealer told me the same thing! That said, the fluid level is fine.
I just drove it again today and it's definitely a 30-35mph vibe and happens in 2nd, 3rd and 4th gears at that particular speed.

 

Is it only when shifting gears?

 

NO! It correlates with a 2nd to 3rd shift at mid throttle, but upon varying shift points with different throttle input the vibrations still lie within the 30-35mph zone no matter what shift pattern is taking place. The only way I get smooth accelaration without vibes is to literally put the pedal to the floor and blow through the 30-35 zone. I had an experiment in mind, tell me what you think: accelarate to 40mph, shift into neutral, coast through the "vibe zone" and see if it's still there, pull over to a complete stop before shifting back into drive. Would this eliminate some potential sources? Also, how does a shadetree flush these trannies? I get a little less than a gallon by draining it and thought maybe about just draining and filling over and over until the fluid was nice and pink again. Lots of questions, I know...thanks for answering what you can!

 

Well you can do the drain and refill if you want.
Or you can get a clear hose and attach to the cooler line at the radiator and start the car and drain till you see bubbles in the hose. Then top off with more fluid and do it again till the fluid is clean coming out.