GEARBOX ISSUES

I have a 57 plate jimny. My gearbox has always sounded a little dry but never thought anything of it. Only could hear it when the rubber cover was taken off. @ weeks ago i noticed a slight grinding noise in 3rd and 4th and a whining noise in all the other gears. It selects the gears fine. Ive now dropped the box and drained the oil out. There were no bits of teeth in the oil or bits of metal. I then split the box and checked the bearings they seem ok just a slight noise but nothing that i would say causes a grinding in 3rd and 4th. I lifted the jimnys back end and ran the car in 3rd an 4th with the wheels off the ground and the noise seem to come from the very back of the box which made me think bearing but I'm still unsure. Has anyone had a similar experience?.. Any help or advice would be great . Thought about buying a second hand box but can't really find one and i don't want to pay £1200 for a second hand one from 247 spares. Also is there a way of making an old g13 box fit where a r72 box should go.