gearbox problems

Hi
I have a 5 speed manual trany from a TD42. It makes a kind of grinding scraping noise in 5th and a little in 4th but only when no load on gears as when cruising, accelarating or decelerating theres no noise.
Anyone got any ideas whats worn out?
Also have the tranny on the bench but cannot get the 5th gear off the counter shaft, looks like it is just a splined shaft, anyone got any suggestions
Duramax

1st problem - Synchro's worn, possibly due to towing a trailer, caravan etc for a decent period, OR
oil level not quite right.
Long shot - oil is not viscous (thick) enough. Although wonder why other gears like 1st and 2nd are not doing the same.

2nd problem - I can swap your Duramax for my 5spd (I can even chuck in my 3.0L), the you wont have g/box issues at all. :-)