Escort Transmission - Selector cable?

I have a 92 escort lx auto w/ high milage (just hit 200).
My tranmission is sluggish but I do know that I need to change the fluid.
What bothers me is that the overdrive rarely kicks in these days. It was great when I first got it but now it only works randomly. I used to feel the kick down at 60, 80 and 100 km/hr. Now I will hit 100+ and it feels like the engine is working really hard and rides near 3.5rmp (it's 2.5 when I have the overdrive working). I've been doing some research and I know that it's sort of electronically controlled and it might be a seized cable or a computer issue?? A selector cable is what it is referred to online..
Does this sound common/close to being realistic? Do you know the cost to fix something like this? Would old fluid have anything to do w/ the over drive not kicking in?
I really appreciate anyone who can bear all these questions and give their thoughts :) Thanks.
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