Need some advice about my transmission?

I have a 1989 chevy s-10 4.3l 2wd auto. It has 93k miles. My transmission has always shifted well until today, It struggles horribly to shift and then when it gets to 55mph it wont switch over to the next gear. It does it in drive and overdrive. The transmission fluid looks alright its not burned, im wondering if it could need a new filter or something. Any advice would be appreciated.