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.