Has anyone tried installing the Ulmo's stash mod? Does/can it work through the gog version? Does anyone know how where to get it and how to install?

If it is possible, will it let me retrieve items stashed when playing a different difficulty level from the one i was on when i stashed them?

Thank for any and all input!

I have not tried it. I expect it would work with the non-widescreen version, as that is almost identical to the v1.09 Diablo for which Ulmo's Stash was made. It will probably work with the upscaled version too. I don't know where to get it, and I don't have a copy.

I expect it would not inspect the difficulty level. If I recall correctly, it overrides Gillian to implement the stash's user interface. Checking the difficulty level would be extra work (not much, but more than nothing), and would only burden the user, so I cannot see why Ulmo would have included such a limitation.

Haven't tried the solution with the GOG version, but yes, it is a global stash. All of your characters have access to it.
If you don't get it going, you could also try Diablo Torch (which is the other of the two stashing programs I know of): https://freshmeat-blog.de.tl/Diablo-Torch.htm