First of all, the transmission fluid pump is part of the transmission. In most auto transmissions it sits right behind the torque converter (which is attached to the back of the engine). In order to change one out, you need to remove the transmission. It is not something which the average person (or mechanic) can or should do. The reason for this is because if the pump is going out, this usually means it is self destructing in one manner or another, which means there are metal flakes from it circulating (at least until it hits the pan) throughout the transmission. This means the transmission is going to require a complete tear down and clean-up in order to fix it correctly. Since someone is in there cleaning it up, it's an opportune time to actually rebuild the transmission. A large chunk of the bill for repair is going to be the labor of removing it from the vehicle and such. If you have the means and ability, you could save yourself the large chuck of change by pulling and installing it yourself.
With all that said, I'm not so sure it is the pump. From the sounds of it, you have something else going on with the transmission. The noise, lack of drive-ability, ability to drive for a bit after removing the connection to the "sensor" leads me to believe it is internal to the transmission and will require a complete rebuild.
There is a problem there which is not going to go away on its own. If you continue to drive it, you run the distinct probability of damaging hard parts (if you haven't done so already). The more hard parts, the more expensive the rebuild is going to be. (Hard parts include things like gears, planetary, sprag unit, etc. Soft parts are things like the bands and clutches.)
