13 speed stuck in high range

2001 w900 stuck in high range, where do I start? I dont think its anything mechanical as the transmission shifts fine and doesn't make any noise while driving. Sometimes I can get it out of high range but it will just get stuck in neutral, it never goes into lo range, but I can flip the selector up and hear it click into high range. I have a load to deliver Tuesday, dont think thats happening now.

 

The transmission was rebuilt then the truck sat for 3 or 4 years. Pulled it out of the parts truck and has been in this truck for about 6 months

 

I took the plunger out of the housing not rusted, the o rings looked fine not torn or missing. I put air to the housing with an air gun, I can hear it going to low and high range. Took it for a drive and it went into low range once, but at the next stop sign it got stuck in neutral and was only able to get into high range. Does this seem like a weak air valve?

 

I used shop air at 120psi. I think im going to replace the slave and splitter valve tomorrow. Everything is closed around me today