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?

First off, the 1986.5 Hardbody is not a "720". The 720 line ended with 1986 model year. Hardbodies were introduced halfway through the year.

The "interlock" is for the clutch, so you can start the truck in gear without pushing down on the clutch pedal. Nissan did not put an electronic locker into a US built truck until the Titan came along for 2004.

The whine is most likely the rear end as you have already figured out. With that much mileage on it I would say it is normal. The pitch and intensity of the noise will rise and fall with vehicle speed. You could try a somewhat thicker diff fluid to see if that helps.

Awesome! Thanks for the information and for straightening me out.