I had a similar issue in my 1982 244 MT with overdrive button. Sometimes it would work, sometimes it wouldn't so I assumed it had to be an electrical issue. After checking the button and relay for continuity (both were working fine) I decided to tear into the shifter.

After I had everything apart, I noticed that one of the two wires leading to the OD button was split in two different places. I rewired both wires (just to be safe) and now the OD is working perfectly!

Not sure if this will help you, but before you go out and buy a new relay/switch/solenoid you may want to check the wiring in the shifter and below it. It definitely saved me money and headaches.

BTW: while you have the shifter apart, take some WD40 and spray the lever that allows entry into reverse (spray the spring and the shaft that the spring goes around). Mine was constantly sticking (up or down) and after greasing it it works like a champ!