My first tranny completely went out so I went to the bone yard and got another for $400.
The junk yard tranny on my 84 300DT started doing the same. It would rev up to 5000rpms at around 40-50 mph. It would then not shift out of 2nd gear. I dropped a quart (the fluid was pretty brownish) and added a quart of trans-x.
The rev problem went away but it still hesitates to go into higher gears the first few times it shifts. I have only driven with the trans-x for 10 miles so I don't know how long the cure is for.