1991 Chrysler New Yorker

I have a 91 Chrysler New Yorker. As I was driving to the store the other day I noticed that it wasn't shifting up as I accelerated down the highway, and the engine started to overheat, so I pulled over and checked the fluid levels. They were fine except that the transmission fluid was on the dark side. Is the transmission going out or just in need of a fluid/filter change?
April 21, 2007.

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