Not clear where it is froze up.
Is it froze up at the top - inside the rear end housing or is it froze up at the bottom - externally where the pedal pivots on the axle?
If internally I don't know what would cause it to do that. It is a pretty simple mechanism inside. Perhaps the shaft #20 is froze up from rust, etc. If that is the case you might free it up by drilling a small hole in the cap/plug #23 and squirting penetrating oil in there. Worst case you might need to pull the axle housing (trumpet) to free it up. I doubt you can do anything to it from the top cover.
If it is the pedal pivot on the bottom of the axle that would be easy to remove, free up and grease.
I am going up to my property tomorrow. I plan to remove one of the trumpets on a scrap 3000 rear end to use as a pedestal for another bench grinder I bought. Can't remember if it has diff lock on it tho. If it does I could remove the right trumpet and sell you the linkage. But even in a USPS flat rate box shipping would push your cost to most of $40.
My email is open in Classic View.
Those trumpets make great pedestals for stuff :)