I have done some search and destroy in trying to get some dtail on a certain part. Correct me if I am wrong but I think it is the speed sensor. It is located aft on the transmission on driver side and has an electrical plug attached to it. My dealings with this particular part came to be when fluid started leaking from it. So 2 questions:

What is it?
Do I replace the whole thing?

 

That's called the VSS (Vehicle Speed Sensor). It does electronically what the old gear driven cable drive speedometers had.
The cars computer also uses info from that sensor to control other things like trans shift points, cruise control, etc.
As long as the sensor or housing isn't cracked, you should just need a new o-ring.

 

is there a part number for that oring or find one in the do-it-yourself section?

 

any tricks..or you should have's about changing that o-ring or removing and re-installing? Sorry for the questions but, I am not a fan of transmissions and try to stay away from them as much as possible.

 

VSS O-ring
The sensor is held on with one bolt. Remove that and gently pry & wiggle it out.
Unplug the harness if it limits your movement.
To avoid losing trans fluid, park the car on an incline if you're able, so that it's higher on the drivers side, and/or jack up and support only the drivers side.