Q: Vehicle is shaking bad, loss of response, slows down when off accelerator almost like it has an engine brake. Starting to lose pwr

I changed the transmission fluid and filter,degreased the engine and did the timing. Put it back together with the proper torque specs. I added dextrin 2 and Lucas transmission additive. I took the car out for a drive, everything was fine, shifting gears very good and responsive, but once I got on the freeway (55+mph) the car was shacking very very badly. I thought the wheels were about to fall off. I got it home, next time I took it out for a short spin the shaking was still there, but at an even lower speed (40 mph). I checked the timing, it was fine, CV axles seem alright, the bushings are all good aswell. So, I took the car out for one last putt this time the car was shaking at (15-25mph). I've looked over the transmission the only thing I found odd was a black ground wire that wasn't attached to anything, but it looks like it hasn't been used in a long while. It just seems very strange that this happened right after changing the ATF and filter. Do you think it could be a faulty transmission solanoid? Any suggestions?

My car has 300300 miles.
My car has an automatic transmission.