92 voyager wont shift into OD

Iv had the van couple weeks in this time gave it full tuneup, new tires balanced etc.. And while giving it gas it has a bit of a shake then clears up when i get to cruise speed. Then just today coming home it wouldnt shift into OD im guessing its OD it not going into since i can run 60mph and the motor only sounds like im passing someone. Not to high sounding rpm's. The van has the 3.3 with 217k miles. Im wondering if it could be a major problem or just mabey a fluied filter change? Any ideas to mabey test to eliminate problems?

$20 for filter and fluid so im gonna try that first.

Well when i took the pan off the magnet was double the size with crap. Cleaned it all up put the filter and fresh fluid in. Took it around the block and it shifted fine actually seems like it shifted alot smoother than before. Like when i would put it in gear from park it was stiff and did a faint clunk now it isnt doing that. But the real test will be tomorrow when i take it to work.