1984 300TDT with 319K.

Symptoms - Shifts OK between 1st and 2nd with a slight clunk. Slips when going from 2nd to 3rd, much like a mis-shift on a standard transmission, but then goes into 3rd. No problem from 3rd to 4th. Moderate clunk downshifting. Clunks seem to be more noticeable than before the problem appeared. This happened all of a sudden. I had a significant clunk problem a few months back, which was resolved by replacing a 2 inch piece of vacuum line.

Does this appear to be vacuum related? Or do I need to look elsewhere?

 

If it were mine, the first thing I'd do is a transmission service.
Check for a burnt smell in the fluid and debris at the bottom of the pan and in the filter.
What is the working pressure in the transmission?
At 300+, she may be getting tired...