Noisy Front End

It's a '99 XLT 4.0 4X4 with about 150K miles on the clock.

The noise sounds like it's running in second gear with knobby tires.
It isn't.

I've done some Google searches, and have managed to eliminate the
following:
It isn't the fan clutch -- the noise varies a little with vehicle
speed, not engine speed.
The tires have about 50,000 miles on them, and are wearing evenly with
no cupping.
The 4X4 shifting works as good as usual -- a bit sluggish from non-
use, but it does shift. The O/D works. Threre are no codes set in
either 4X4 or O/D.

Now, some guesses -- my "wonderful" Haynes manual tells me that
earlier models had fill/drain plugs on the front differential, but
says nothing about the later models. Does my front differential have
fluid fill/drain plugs? I've spotted one that MIGHT be -- it's
located on the rear of the front differential, and just a hair above
the axles. I'd really like to check the fluid level -- though there's
no sign of leakage.

How about the hubs? Will they cause a noise like that? Is there any
way to inspect/troubleshoot a hub without a complete dissasembly? If
a hub is bad, can I replace just the one that's bad, or do they need
to be done in pairs? It's a financial consideration that would make
replacing them a month or so apart a whole lot easier to swallow.

If it's a hub, how much time do I have (guesstimate) before it leaves
me sitting on the side of a road somewhere?

Any pointers?
TIA,
Mark