lsd rear end

my 2000 xterra has a limited slip differential. however, to satisfy my curiosity, i wanted to know how well it engaged and at about what speed it started to engage, so i jacked up one wheel and put it in drive, and lightly reved to motor. to my surprise, it never engaged, why would this be
thank
mike

You don't understand how an LSD works. There is no "speed" at which it engages, in fact it's engaged at all times until it slips. It's a proactive device, not a reactive device, better at keeping a wheel from spinning in the first place than stopping one and/or sending power to the other side once it does.
The LSD on the 2000 Xterra was tuned VERY weakly, don't expect to be able to drive off your jack with it or anything. When I was testing mine out a while back I had both rear wheels off the ground and I was able to hold one tire from rotating with my shoe when a friend put it in gear, the other side would spin happily.
I imagine you heard a dull ratcheting-like sound as the lifted wheel was spinning helplessly, that is the sound of the LSD clutch pack slipping.

Brent

Info on how to tweak/tune the LSD if you're mechanically inclined:
http://nissan4wheelers.com/eve/ubb.x?a=tpc&s=227600023&f=728603133&m=5276043182

There are no aftermarket LSDs for the H233b, the only traction aid available is the ARB air locker.