Can somebody tell me what's wrong?

the day after i recieved my car from the shop, i noticed that whenver i go above 50mph and my RPMs are low, the RPM meter drops and locks at 1400 and no matter how much gas i give the pedal, it stays locked. if i continue doing so, the car begins to shake.

It doesn't happen when i make my rpms around 2000-3000, it's only when i try to coast at 50 or above. It happens when i go anywhere above 50 mphs, liek say i were going 70, if i tried to coast, the rpms would drop down to about 2000.

The shop just fixed my transmission, is this a transmission problem? i really don't know much about cars so do any of you guys know?

There is a cable or a vacuum line that is supposed to tell the transmission when to take the car out of the locked TC mode and when to downshift. Take it back to the shop and have them hook it up, or if it is hooked up, tell them to adjust it correctly.