For the collective -- '99 XLT 4.0 with about 150k miles.

Within the last few weeks, a roaring noise has developed in the front
end.  I have Googled quite a bit, and can eliminate a few things that
are probably not causing it.

It is not the fan clutch -- noise is constant as long as the car is
moving (above about 20 mph), and doesn't vary with RPMs.

It's probably not the tires -- they have about 60,000 miles on them,
and are wearing evenly with no cupping.

The noise sounds like I'm running in second gear with knobby tires.
I'm not.  The O/D switch functions, and there have been no 4 wheel
drive or O/D codes.  The transfer case functions -- I can switch to 4
high or 4 low with no more trouble than usual (sluggish because it
isn't used often).

Now then -- suspicions -- my "wonderful" Haynes manual tells me that
'94 and earlier models had a drain/fill port on the front
differential, but makes no mention of later models.  Does my '99 have
a drain/fill port?  I'd like to check to see if it has enough fluid --
even though there are no obvious leaks.  There is what appears to be
on plug on the rear of the differential, just a hair above axle
height.  Is that it???

Next -- if that checks out OK, is there anyway to check the front hubs/
bearings with major dissasembly?  If that's what it turns out to be,
can I replace the hubs one at a time, or must they be done in pairs?
The financial consideration would make replacing them a month apart a
much more viable solution, but I'd like to fix whatever it is before
it leaves me on the side of the road someplace.

Is there anything else it could possibly be?  Help me out here!
TIA,
Mark