On a couple of my cars that came from Pa. (where they salt the roads in winter a lot) the shifter shaft going into the auto. transmission got corroded with the aluminum of the case. I had to drip oil (ATF) onto the shaft with an applicator that would reach under the neutral-safety-switch. This enabled me to keep using the cars at least.
If you have coolant getting into the tranny, its probably coming from the ATF heat exchanger inside the radiator. It could be that your least expensive course would be an ATF change and a new or repaired radiator, or installing an auxillary oil cooler. Sea-Foam makes a product with a name like 'trans-tune' that might help the inside of your tranny a little bit; you add it to the tranny fluid, after you have fixed the coolant-contamination problem.