Meritor 10spd trouble shifting from low to high

Might have inadvertently knocked loose the small air line or fitting that goes into the transmission? (comes from the shifter to the trans.) If that's the case you should be able to hear air leaking (while in the high and/or low position). If it works "partially", it could be a minor breach in the air line.

 

I'd back this up, that line also runs under the cab directly to the trans as well.

Shut the engine down and flip the selector up and down and listen for an air leak.
Didn't have a meritor, but I've had this issue hit on an Eaton in a FL. Not audible in the cab, very audible outside with the engine off.

 

I drove a truck with the same trans and had the same issue. It wouldn't go up to the high gears and also wouldn't go down to the low gears. Company had it fixed. They told me the trans was shot. Don't know if they got ripped off or not

 

Range synchro's.