So this morning I was driving to work down I-65 doing about 75mph and I had this horrible shake/shudder/vibration only when my foot was on the gas, but as soon as I let off the gas it would subside. It does it the worst over 70mph but it starts to be noticable around 55mph.
Sounds like what jr'* 3800 I believe is looking for. The torque converter clutch solenoid is acting up. I had one go bad on a previous car & it done the same thing as you are describing. If you try like jr suggested by driving with the car in 3 instead of D & it stops that will most likely be your problem.
Also watch your tack wile it happens. You will see it jump back & forth a couple hundred RPM from the TCC engaging & disengaging.
Honestly, I really haven't put any money into mine yet except for appereance things, but me and my fiance are closing on our 1st house tomorrow morning and obviously have no spare money with Christmas and such. Atleast she'* parked for the winter 99.9% of the time. I only drove her today because she had gas and my t-bird didn't.