1990 RHD TLC 80 Series doesn't shift into overdrive. What can I check for self diagnosis?

New to this and a Long one but here it goes....I purchased a RHD 1990 80 Series TLC for my wife last July (73,000 miles) who is a rural carrier for the USPS. I've read a lot of older threads about different topics in here that have been very helpful. Our current situation is this: vehicle has approximately 110,000 miles on it and it seems to have lost overdrive. Will not shift into OD once you reach 55-60mph like it used to. Runs around 2800 RPM between 55-60mph. I have changed the tranny fluid and filter with no improvements. When first purchased I had also adjusted the throttle cable a bit (so it would not shift into last gear while doing around 40mph in town), new distributor cap (old one was cracked), rotor, plugs, plug wires, fuel filter, oil filter, tranny filter & complete fluid changes were done before I put it on the road for her to abuse everyday (her route is 140 miles a day w/120 miles of it all gravel roads). The performance improved quite a bit after those maintenance items were completed. Real solid vehicle for the conditions here in Iowa and especially this past spring with it being the worst in 30 years or more around here. Extreme frost heaving on the gravel roads (like driving in quicksand) burned up the transmission in her backup vehicle 96 RHD Jeep. I also replaced the front end steering knuckle seals, bearings, and gaskets as one started to leak quite a bit. I am mechanically inclined, electrician by trade, but I AM CLUELESS when it comes to transmissions because I've never worked on one. We live in a small town (3,500 people) with only a small independent guy that does mechanical work that will be retiring in the next few years and he's not much help on something big like this. I've checked around the transmission for broken connectors, wires hanging, etc; and cannot find anything out of place. Is there anything anyone in here can tell me to check or others that have had similar problems. A440F tranny from what I've heard is bullet proof but I wanted to check here before I justify dropping 4K on a new tranny. Any tips would be greatly appreciated. Thanks.