10 Speed Eaton wont shift into HIGH

Sounds like a synchronizer issue, broken pin or something like that.

If the range piston on the transmission is cycling when you flip the range lever on the shifter then you're likely either going to have to pull the aux section or the whole transmission, depending on access.

Definitely recommend heading over to Eaton's website, they offer their shop manuals for free download. There'll be some good information in there on troubleshooting.

 

You won't see much of the aux section with the top cover off. Eaton Roadrangers are more or less 2 transmissions in 1 housing - main box in the front and an air operated aux transmission in the rear.

Hard to say not being there in person but it sounds like your issue is likely related to the rear section, either with the transmission air system or an internal issue with the range synchro.