1995 Mercury Grand Marquis Starts but Won't run?

I have a 1995 Ford Mercury Grand Marquis. I got it with only 40,000 miles to it's name and it is in great condition. I haven't had one single problem with it since I have had it and it has been running like a pro. Then one day the check engine light came on. I assumed it was low on oil so i got an oil change. Still came on. So i figured I would drive it and see if I can notice anything strange that I could help diagnose the issue if and when it needed to be checked out by professionals. Then all of the sudden without any prior warning AT All (AKA no sputtering, no clanks, no issues starting, no strange vibrations) I was at a stop light and when the light turned green i pressed on the gas and the car went but did not accelerate, it was like i was trying to drive with the emergency break on (No, it was not on). The thing starts fine but now it won't accelerate faster than 10 mph and thats with the pedal to the floor… I took it to the shop and they have no idea what's wrong with it… They diagnosed it with a test and it brought up pretty much every electronic device within the transmission but the experts say they don't know what Exactly is wrong with it… The transmit ion seems to be fine. Plus it happened so suddenly it was like somone unplugged the cord to my horsepower. The people at the Auto Shop don't have an exact clue so they are referring us to a transmission specialist but that isn't until next week… So any ideas what it could be so I don't have to wait that long?

Added (1). Oh and Forgot to add that when the car is in drive and I press on the gas the engine revs… And when you press the part inside the engine (sorry I'm not a car expert so I don't know the term) manually by lifting the hood ( I'm guessing this is the part that is related to the gas petal signaling the car to accelerate)… The engine sounds beautiful. But pressing the petal, it won't accelerate.

Sounds like you may have a plugged up catalytic converter. No power or acceleration under load (driving), but will rev fine in neutral. Have that looked at.