Id recommend this.

Drop the gearbox oil
(in heinsite, if valvoline, the most common gbox oil would be 85w - 90).
Disconnect the fuel pump relay, so that the engine will not start.
Crank the engine for a max of 10 seconds, that should evacuate 99% of the oil in the box. ( make sure to maintain a drip tray under the open gbox drain plug, to catch the oil ).

Once most of the oil is drained out, you should put in a heavier oil. Like a 110 - 120w gbox oil. Make sure not to use a gl-5 or gl-6 oil, as it will corrode your internals. In the gbox. ( the chemical components of these oils are made for LSD diffs, and worked gboxes.

Put this oil in, and make the slowest shifts that you Can, for at least 2 months. Drive like a grandma. Hopefully you might get rid of the crunch. Works sometimes, not all the time.
But it's deffinately the synchro's!

Not cheap to fix, but if you can put up with it for now, it should be all good.