I'm willing to bet dollars to donuts that your diff lock no longer functions properly regardless of pedal position.  If possible, safely jack up the rear of the tractor so one wheel is off the ground.  With the engine OFF and the gearshift lever in neutral, rotate the raised rear tire by hand, it should spin freely.  Then, continue to rotate the tire while pressing down on the diff lock pedal.  When the diff lock pin goes into place, the raised wheel should no longer be able to be turned.  If you can still turn it, you have some broken internal parts.  Because you stated that you have no need to use the lock in the future, this may not be a problem you have to fix.  I would recommend changing the fluid and filter though.