From what I understand, the Xterra uses a H233B Diff with 31 splines...
From the Calmini website.

Lock Right positive locking differential is second to none for extreme traction off-road. The Lock Right unit's positive locking action allows for lower approach speeds to obstacles which means more driving control and reduced tire wear. Installs using stock differential case. No special tools required. Complete hardware and installation instructions included.

Fits 86.5-2002 H233B 29 or 31 spline axle

Part Number NP3220 Price $269.95


Detroit Locker®
The most rugged positive locking differential, and most widely available for axles from 3,000 to 70,000 lb. capacity.
CALMINI offers the Tractech Detroit Locker in in 31 spline for late model Xterra V-6 Models.

For severe operational conditions.
Driver may hear and feel "backlash" going from drive-to-coast and coast-to-drive, as reversing torque flow resets the components.

Part Number NP187s/128 Installs in stock open diff case Price $449.95
Part Number NP187sc/128a Replaces stock LSD diff case Price $549.95


My question is what if you have an LSD. Will thw powertrax unit work with that or is the Detroit the only one available?

There really is no application chart, and I know most who wheel don't have an open diff because most Xterra "packages" that come with 4x4 also came with LSD.