Yes, I forgot I do it in reverse gear sometimesThe diff lock pedal on the floor isn't stuck in the down position is it?
It's been a long time, but do I recall correctly that it may be the shift forks getting stuck?
Have you tried @razerface suggestion but in Reverse?
That's what I''l do. TYProbably just rust then. Change the gearcase oil to super udt and just work it back and forth a little every day as much as you can till it breaks free