I’ve explained to quite a few people how to bolt a length of 2 by 4 to a gearbox to be able to keep the thing from rotating as you go to loosen or tighten the big pinion and input gear nuts. Or bolting the box to a stout bench… If you don’t have air tools, or don’t want to use impact against the gears, you can get an easy grip on the case to apply lots of force GENTLY.
Here, I’ve locked the box in first and fourth. I did not capture how to do this on the rod-change linkage… you must rotate the shift stub shaft out of engagement to be able to move the shift-forks independently, then manually slide the hub collars to engage 2 gears at once. As all gears are in constant mesh, nothing can turn with two different ratios locked. Now if you can grab the box securely, you can apply the necessary pressure gently to get it (un)done!
If anyone wants to see the linkage trick, leave a comment! I probably have another case I could show. Specific questions make for good relevant content, so ask away!