I had never encountered on of these before and didn't even know this was a thing:
https://www.bigsquidrc.com/team-asso...l-hex-adapter/

However, now that I've seen it, I'm very interested. I don't quite want to spend $100 bucks on one, so I'm wondering if anyone has cobbled something together at home that performs a similar function. I have a 3d printer, I can solder / run wires, etc. Not great a designing electronics (i.e. if there's more to it than plugging in a few wires, I'm lost) but I can handle mechanical things ok. Maybe there's a way to make a mechanical version of this that just has a spring scale style needle? I don't need high precision - just curious to see how my diffs perform over time.

Thanks!

Checkout this: Made a diff checker

Thank you!

No thanks!
The idea is simple, the more stiff the diff is the more current the motor will draw to rotate the diff. Because every brand diff checker has its own setup you can not compare with others or take it up in an universal setup sheet. But for your self you can keep track on your diff if it is fading or when you change it to see if it was really changed. Beside that, most drivers also use it as a warm up for the diff. Before driving rotate the diff for about 30 secs it is "on temperature" and so more constant from start to finish