Getting P0500 - already swapped trans speed sensors to no avail

My 2001 Jeep Grand Cherokee with the 4.7L motor is running really rough and eventually
stalls.  Originally I pulled codes for misfires so I changed out all the spark plugs.  Started up
the car and it's still doing the same thing.  Idles rough and eventually stalls.  After doing a
search on the P0500 code I went out and bought one speed sensor since both sensors are
the same.  I swapped out the output sensor, started the car, same thing.  I took the sensor I
just took out and swapped it with the input sensor, started the car, and still same thing.

Autozone also had a vehicle speed sensor but I think that's for the v6 version.  It looks like a
white plastic plug.  Even the thread is plastic.  I couldn't find anything like it on my car.  

I've read some jeeps have a speed sensor on the rear diff but I didn't see any sensors
connected to it.

What else should I try?


Thanks