Differential Questions

[pharmer <pharmer@domain.elided>]

Folks,

I think I might need a new R&P for the rear axle in my Travelall,
and I've got a few questions.  

Here's the deal - I noticed a groaning noise back there about a month
ago, checked it out, and sure enough it had leaked a bunch of fluid 
out from around the plug, and was low.

Had a mechanic check it out, he said it was fine, but a week later while
getting new exhausts installed, the exhaust shop noticed that the plug 
was completely missing, and the fluid was very low.  My wife, sounding
kinda desperate I assume, called up GUS, which was just around the corner, 
and those guys dropped off a used plate & plug for nothing.  Side note: 
one of the reasons that I enjoy my IHs so much, is that it gives me an
excuse to hang out with such fine people!  Anyhow, the prior mechanic
checked it out again, said it looked fine.  

BUT...now I can hear a low-pitched whining sound from (I think) back 
there.  It isn' too loud, but it wasn't there before, and I assume it 
will only get worse.  Also, the noise depends on the axles/wheels/diff
not the tranny/engine since it depends on speed and not engine rpm or
transmission gear.  I suppose this could be a coincidence, and maybe
its a bad bearing, but I suspect not.  Does my diagnosis sound correct?

Assuming that it's the differential, and assuming that it will be going
out in the next 5,000 miles or so, what would need to be done to
fix this - replace ring and pinion?  I've been quoted labor costs of
around $500, along with parts costs of around $200 to do this.  Should
I go with someone who specializes in this, or is the work simple enough
for a generalist?  Any other work that should be done if I end up doing
this?  Any ideas on a silver lining here?  I'm not really interested in
a gear ratio change (I've got 3.73, and tow a camper thru the mountains).  
Would this be a good time to get an ARB installed, and save a bunch on 
labor?


Thanks,

Ken Farmer
1980 Scout II
1975 Travelall
1974 Travelall, 392, NP205, TF727, 3.73 r&p, Dana 44, 30" tires