I have a 1986 F-350 with the 6.9 and a C6 that I was told was replaced 15k ago. I've had it for three months and driven it a few hundred miles, both town and highway driving.

I live on a hill, and when i was backing in with half a load of firewood, the tranny started grinding. What could be the issue? Is it something that could be fixed easily, or is it time for a new tranny?

 

Are you sure the grinding was the tranny, was it slipping also.
Is it still doing it after you unloaded the wood.

 

Once I emptied the wood, it did fine in forward and reverse. No slipping. Last night, I did the same thing with half a load and it grinded a little again.

 

If It is not slipping, I am leaning more on the carrier bearing, a ujoint, something that is changing while you are under power in reverse.

 

I would pull the pan and check for shavings, but I doubt there will be any. I would be checking the rest of the driveline.