Your OD problem, refusing to go into OD (4th gear), is probably not a transmission problem.

You have to know that the "default" condition of your transmission is to only use the bottom three gears -- i.e., if there's anything wrong with the electrical solenoid controls, the transmission works as a ordinary 3-speed transmission. To go into 4th, the electrical controls have to work perfectly. Unfortunately, on the 240s, there's typically a few potential problems that can interfere.

First and foremost is the control relay (behind the right vents in the center panel, very easily accessible by removing the glove box and just sticking your hand in behind the vents. Do you hear a click when you push the button (next paragraph)? You should if everything is working correctly. Does the light on the dash go on?

Second, the switch (small button) on the shifter -- sometimes its contacts get corroded and the switch won't respond to your button pushes.

Third, a simple wiring failure (failed insulation causing a short or grounding).

Only rarely is the solenoid (for which the IPD plate substitutes) responsible -- and this is the hardest thing to work on or replace, so it should be the last thing you try.
If you hear a click when you push the button, and you see the light go on or off, and yet your transmission doesn't shift down or up according to the light (respectively), only then should you blame the solenoid.