Ok subject is 65 barracuda with 360 4spd w/od . I have a vibration in 4th gear at about 3000 rpm and up. In neutral all is good. I thought it might be drive shaft related, had that checked, all good. I just recently noticed 1 2 and 3rd gears good until going into 4th. I'm guessing since 4th gets probably the most drive time something worn there. Thoughts ?????