Re: Rear end question
Does it howl at lower speeds, any grinding? even momentarily? Any axle binding or abnormal feedback from the steering wheel?
Drain plugs will be at the bottom of the pumpkin. If it's anything like my Dodge there's a fill/check hole only, and in order to get the fluid out you'll have to siphon and then remove the diff cover to clean the magnet. If you notice gear whine when you push the go pedal you might have a more serious problem than low oil.
Take the plug out and the oil level should be right below the hole. You'll have to check with a finger. If the oil level is fine I'd remove it (change the fluid). I'd also remove the covers and check the magnets for any debris (metal shavings etc). Gear whine can mean several things, anything from the ring & pinion being on its way out to a bad clutch pack (LSD only, if so equipped), or a bearing pack (most common). While your under there be sure to change out the transfer case fluid as well if you haven't done so in the last few years or 40K.
A better case scenario (but still not the greatest) is the possibility of a bad wheel bearing. They get louder with speed the more its worn. No bearing breaks down the exact same and can be easily misjudged, so it's hard to pinpoint the source of your 'howl'.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!