As my reader will know, my mechanic suspects that there might be a leak
between the tranny and the differential on account of the level of oil in
the differential has been dropping, and the fluid in the (automatic)
transmission looks a bit discolored.

If there is a leak, it's Bad News.

One way to investigate this is to separate the two and inspect/replace
seals. The quote I got for that is $350 to $400.

Dennis Haynes suggested an alternate approach: get an oil analysis done to
see if the two lubricants are getting contaminated with each other. That's
$10 for the kit, postage extra.

This strikes me as a more affordable approach.

Bentley's doesn't show (that I can find) where the filler plug of the
differential is. So my question is: what's the easiest way to get a few
drops of differential oil out of the thing? Special tools needed?

--
Mike "Rocket J Squirrel" Elliott
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
Bend, OR
KG6RCR