From deceased epic contributor, jinman: ..when you step on that pedal, it moves a shaft attached to a shifter fork inside the differential case. The shifter fork moves a "clutch" with four fingers to mate with the differential gear case and lock both axles. If the differential lock is engaged while the tractor is moving, those fingers can be damaged or bent. All you will do by forcing the pedal or shaft is to do additional damage. If you can't see something binding on the shifter shaft externally on the linkage, there is certainly nothing inside that is going to be cured by slamming or hammering on the engagement lever or shaft. Differential lock works smoothly or it is broken/malfunctioning. What you are doing by forcing the shifter is the same as trying to force a shift lever on a standard gear transmission. You will only make the problem worse internally or break the external shaft in the process. You will have to go into the differential and fix it or just live without differential lock. I'm pretty sure the former owner must have engaged the differential lock while the tractor was moving or turning and messed up the lock clutch fingers.
https://www.tractorbynet.com/forums/parts-repairs/268827-tc33d-cant-engage-differential-lock.html
