Followup: that's exactly what it was.  Since you put the thought into my
head, I stopped as soon as I got home before I even went into the house,
reached behind the gauge and pulled the bulb out, et voila, back to
normal vacuum readings.  So for now I will leave the bulbholder
partially out of the gauge and tape it into place, I think the real
"fix" is to replace the gauge.  The whole reason I replaced the bulb was
because I was using a cheap "sunpro" parts store gauge, and found that I
couldn't see it at night, so I replaced the bulb (looks like a 53?) with
an 1816 in an attempt to be able to read it.  The 1816 is tubular and
apparently longer enough to catch something in the gauge mechanism.  I
don't like the backlighting at all in the Sunpro gauges, the one VDO
gauge next to it looks so much better at night even though in the
daytime they are near identical in appearance.  Lesson learned, pay the
extra $10-20 for good gauges.

To the other poster, I really didn't expect the vacuum gauge to help me
with economy (economy in a full-sized pickup?  heh.) I added it for
primarily two reasons: 1) to keep an eye on engine condition and tune
and 2) because I wanted to add an oil pressure gauge and a trans temp
gauge, and I already had a three hole gauge panel that I wasn't using
for anything and buying a new two hole panel was about the same price as
a cheap vacuum gauge.

nate