F-350 diff whines...

A '96 F-350 has a whine from the rear end on acceleration only.
It is 5.3L gas/auto transmission.
Since it's only when accelerating, might the diff be be 'dry'?
If so, what is the type of lube he should put in there.
I do not know if it's 'limited-slip' or not. The owner doesn't, either.
This thing belongs to a friend who does no DIY, so I'll ask what's the size of the
fitting to remove the plug, a regular 1/2" socket drive?

Almost all the time a 3/8 inch tatchet with a 3 inch extension will work. If it feels like it is going to break they split by the ball which holds the socket.
Use a tool like this http://www.autopart.com/tools/toolsmain/tool/T_2049.htm
I have various sizes of these but you will want a 3/8 square to 3/8inch or 1/2 inch drive. You don't get many but if its never been checked they are very hard to remove. Use a small screwdriver and clean out the hole before you start.
Check and make sure of size before ordering as I have 5/16,8mm,10mm,3/8. If a 3/8 inch is sloppy it will be 10mm.

Something else that makes noise back there is the U-joint at the back end of the driveshaft. If the truck has had no maintenance, a dry u-joint will make strange squeaky noises, mostly on acceleration.
By the way, if its low on differential fluid, I think the same fluid also makes its way out along the axle shafts (inside the axle housing) to provide lube for the rear axle wheel bearings. So you should hope it isnt VERY low.