grinding transmission

You'll be fine

 

The inside circle inside the top is friction material kind of like a car clutch. When it wears out they will start to grind. Just some noise, a little hard on the gears, no short term problem. The three silver circles on top are pins. There are 3 up and 3 down. Some times they brake causing grinding, can prevent engaugement to the next range and in the worst cause jam between ranges.
If it is only faint grinding and or you can make it stop by shifting at a different RPM, or clutch in, shift N, clutch out clutch in split then shift, type thing I would not worry about it at all. Get money get home.
If it is hard grinding for and extended time I would empty home in high or fix it there.
R&R back box and sync. in a day.

 

Sometimes all the friction material is gone as if the bonding agent failed. Those grind a lot and that noise is eating up the $450 auxiliary gear. If the pins break it does not matter what gear range, split,or gear you are in the grind go's on. From your continued explanation I'm with Lester. You might try changing how you shift as maybe change range just before going into 11th and or clutch the range shift.

 

Good news is you shouldn't have to pull the whole tranny for a new synchro. Just the auxillary section.

 

The friction material is gone.
When mine was going out,I did 2 more rounds before I put it in the shop.

 

Mine has been doing that rattling noise since I bought my truck and its been 8 years. Long as I split Hi and Lo at 10 mph and under, it wont rattle or "grind" like you say, but I have a RTO 13.

 

If the syncro breaks. You will know it they will make a noise that makes you stop. If you have a constant noise like a rattle there could be a few other things too. The washers that secure the gears to the main shaft and keep them in position can wear allowing the gear to float. It will move just far enough that the teeth on the sliding clutch still touch. What will eventually happen is the gear will eventually move far enough to allow the sliding clutch to be engaged in 2 gears and effectively destroying almost everything with a tooth on it in the main case. You won't hear it in gear but neutral you will. The synchro usually happens in either low or high range and while it is in gear.