Tractor Repair: Case 2290 Diff Lock, heavy duty mechanic, clutch pack

Question
I bought this tractor five years ago - it has been working just fine until this Spring.  I stepped on the diff lock to get out of mud and now it won't release.  I am a heavy-duty mechanic with 30 years experience. However, I have no experience working on farm tractors.  If you could explain HOW the diff lock works I could probably fix it.  Would appreciate any help you can provide.  Thanks.

Answer
  The diff lock button on the floor of the cab operates a hydraulic valve that sends oil pressure to the diff lock piston inside the differential.  The piston squeezes the diff lock plates together like a clutch pack to lock both side gears of the differential together so that both rear wheels turn together instead of individually.  Works great when you're going in a straight line, but makes it near impossible to turn a corner doesn't it?  The diff lock valve likes to stick in the engaged position after you step on it if you haven't used it for awhile.  Try hooking a prybar under the button and prying up on it to pull the valve back up.  You may even be able to pull it up by hand if it isn't stuck too hard.  Spray a little penetrating oil around the valve spool and work it up and down a few times until it will spring back up on it's own.  If the valve is up and the diff lock still won't release, then there is an internal problem inside the differential but it's about a 99 percent chance the valve is just stuck.