got a growling, grinding at speed with clutch engaged, in top gear, but goes away with clutch pedal in and/or in outta gear. already replaced ujoint to fix a constant vibration earlier, but this seems to me like its gotta either be clutch or transmission. i don't know nuthin about either. what can i do to narrow it down?
help.

Overland Blue, 1953 GMC 1/2T, 100-22, 228/1bbl, 4sp/4.10 6v, shortbed/fender side/5-window.