Make sure the fluid is not burt up and you might try putting in a new filter.
I had a Taurus that had the cv joints break. It acted like it was in neutral.
I also had a Lincoln that had a bad tranny. The wheels would spin in gear if they were jacked up off the ground. This is not a good thing to do, but I did it anyway. Be very careful if you try that. Once the wheels were back on the ground, the tranny didn't have enough power to pull the car. The tranny was shot.