10spd eaton grinding sound when dropping from high to low?

Synchronizer is out. There's a brake pad like material on the face of the hi and low Synchronizer, which allows for a smooth transition of the sliding clutch into the aux drive gear (high), or the low side gear.

It needs the Synchronizer replaced. Being an 05 model, it's probably a metric Fuller, model beginning with FRO. The tranny has to be removed to replace it in this tranny due to the range cylinder.

 

I would figure 4-5k if you do the clutch at the same time. The way most techs are now, you'd likely be better off looking into a reman tranny and just doing the clutch while you're there.

If you baby it and only range down at slow speeds, it'll probably last a while still.

 

From what I’ve seen over the years what takes those synchros out quicker than anything is coasting to a stop and making the range shift with the main in neutral. The noise that comes out of them when you do that should really be a clue.

 

Synchronizer is worn out. Call around in your area to smaller local shops that work on transmissions. I had the same issue with a 13 speed I had about 10 years ago. It cost me $2500 to get it repaired.

 

I am quite sure I have had top off some of those in the truck. Used a small bottle jack between frame and right rear of cab. Removed panel under left door and or go in from left rear under cab. No big guys allowed. It is a pita but rolling in less than 16 hours with minimum parts.
Check the rear yoke for play first. One had a good a good bit of play. Thought new bearings would take that out but it did not. Tighten the yoke nut first and see if that takes the play out. Better idea to reman then. Races spinning had worn the housing.
Need to consider usage and mileage on trans as well. Might be better off with new trans & clutch. What about the rear seal an rear housing gasket. Most of our stuff is local and I try to do the minimum on those.