Hey guys, I own an Xterra, too

Anyways, I thought you guys could answer a questions I have about my 1986.5 Nissan Hardbody (720, right?), or at least point me in the right direction. (Probably to another forum

Anyways, I was practically given the 210,000 mile truck, and it still runs incredibly strong. I use it for 4-wheeling and hauling my motorcycles around. It was the top of the line version and had every option including including the 'Interlock', which, if I'm not mistaken, is an electronic rear locker.

Anyways, here's my question. When I get going up to speed, say 40+ mph, there is a whine from the rear of the truck, which I assume is the rear diff. It is proportional to overall truck speed, and not engine speed. Is this normal? Is this the 'interlock'? Should I have anything looked at?