Impossible to move shift selector

2000 Plymouth neon with 86k miles, the automatic transmission has been replaced once with a who knows how many miles transimssion. The replacement transmission has been having problems.

The shift selector is very hard to move. I did a flush with atf +4 and it was better for about a day. The previous fluid was milky looking, and the coolant level has been decreasing. Now it's impossible to move (without breaking something).

I'm guessing coolant in the transmission causing it to lock up. The car still starts fine. Any other possible causes?

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.