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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.