As to having skinny arms, I don't think they are "large" or "heavy", but I'd
like to think they are about average/normal for a middle aged male!  (I'm
5'11" tall and weigh about 185 pounds).

I think that because I jack up and support the frame of the vehicle and not
lift from the wheels/axle/engine, that the spacing at the rear exhaust area
opens up quite a bit.  As I recall, the driver side plug was extremely easy
to access using a simple ratchet, extension, and socket.  The other two
rears were the problem.  As I said, I got at the one from the top by
removing the upper alternator bracket (2 easily accessible bolts IIRC).
Since the alternator is secured by a bottom mount bolt, it did not move and
I did not even remove the alternator belt.  The plug was accessible from the
top with the bracket removed.

As to only needing to do this at 100K miles intervals, I was not so lucky.
My 1999 3.8L T&C Limited developed a rough idle at about 45K miles.  Since I
was a previous owner of a 3.3L Grand Caravan, I first tried using a bottle
of Techron in the tank, followed by replacing the fuel filter, air filter,
and cleaning the throttle body, but none of these solved the problem.  As a
last resort, I changed the plugs and wires and it smoothed right out and we
haven't seen the problem in the nearly 18K miles we've driven it since then.
I don't know if the problem was the plugs or the wires and frankly I don't
care since the effort involved to do either dictates doing both in my book.

I hope this helps explain how I was able to get to the rear plugs so others
may benefit from my experience.

Bob