I've searched and found very vague thermostat replacement instructions, but not being that experienced I don't know where the damn thing is. Generic (non-Xterra) information I've found implies it should be in-line with the upper radiator hose. The only thing I've found that looks promising is in-line with the lower radiator hose. HELP!

Also, if I every find it, how tight do you torque the bolts? The thermostat housing sealer says "not too tight" which is totally worthless.

You found it - it's on the lower radiator hose connection on these motors.

Looks like this (notice the torque specs on the bolts):

Thanks!