I didn’t think I explained it well!
In the first build, I only had 3 out of 4 wheels working in normal 4-wheel drive mode. The diff worked like a dream, though.
In the second (extremely careful) build, I have perfect 4-wheel drive. However, one wheel (the same one that didn’t move in build 1) does not work in diff mode.
Thank you for your suggestion, though.
I’ll give it a go.