Stick with your plan of removing the filter/screen and see if it's clogged full of fibers from a clutch pack. Use ONLY Mitsubishi's special ATF if you can get your hands on some, other wise use Dexron to just get you home until you can drain the trans and refill it with the ATF+ stuff from Mitsu. I and others swear by it for our Monty autos.
As for linkage issues, it should be REAL apparant if the linkage is off or out of adjustment once someone is under the truck and has made sure that the shift is fully in PARK upstairs inside the truck.
Does the overheat light for the trans work? Have you ever seen it come on while driving? If the bulb or temp switch is out, you may have been running hot in the trans for some time and didn't know it <img src="/forums/images/graemlins/scared.gif" alt="" />. Good Luck, let us know what you find out.