just got off the phone with a retired Massey mechanic..  

he  verifies what you said about being hydraulic both ways;   
 when  energized,   the  spool puts  pressure on one side of the piston-  which  slides the shift rail/fork into high..
When  de energized,   the  spool  directs pressure to the other side of this piston.. which slides the rail/fork into low.


He  recommends  me  taking out that  solenoid,  and checking for free travel first, with a jumper wire on the bench.  

he is well aware of my  summer's  problems,  and he'd  just about bet the farm I'll find  garbage that has limited the stroke of that spool.

'Take it apart and clean it".    if that doesn't help,  then  pull the  round cover off,  and  verify the piston's o-rings that move the  shift rail aren't  blown or cut.  

Also,  seeing this  piston is end-of-the-line type thing,   garbage tends to accumulate and could limit  travel.  

So,  I've got a basic  road map now to  follow.