I'm going to install a LSD soon. Let's say I measure the backlash and it's off. Do I have to order an OEM shim from Nissan or an online retailer, or if there a generic part that would work in this case?

Wondering because the turnaround time for ordering online sucks and as far as I can tell it's mostly guesswork to try getting it right.

 

You want to use shims from whoever made the pumpkin and gears. If it's a Nissan product you can call and see if it's an off the shelf item or check
Z1Motorsports.com, gl!