I had similar symptoms and thought it was the 3-4 hub on its way out, but
after taking it to a transaxle specialist found it was only (only?) the
synchro ring. Trans grinds when you try to shift to third or fourth, sounds
nasty. Good news is that it isn't about to fall apart and leave me stranded.
Bad news is that it costs as much in labor to replace the synchro rings and
the 3-4 hub as just the rings, so it's looking like a $800 + repair.

If it's not too bad you might have some luck using synthetic oil in the
transaxle.

Have a transaxle specialist DRIVE it before making any hard and fast
decisions...