1997 Grand Cherokee Alternator Issue

I have a 97 JGC that has the usual dashboard voltmeter drop issue. Lately, that drop has been associated with an actual drop in battery voltage. The problem seems a lot more persistent when it's wet/raining out. I also discovered I can make the battery voltage gauge come up (not necessarily battery voltage, but displayed voltage) come up by taking a piece of wood, laying it against the alternator case and giving it a tap after I start it to make the gauge work right. (No, don't swing for the cheap seats) Now before a bunch of you start yelling "Brushes!", this problem will return every time I shut it off and restart it. Brushes are a mechanical element, and if they're in contact with the commutator, they're in contact. Shutting down and restarting isn't going to break that contact. Something else I've noticed is when you rap the alternator, you can hear it "load up", like it begins to do its job.
My question is, what else is harbored in the alternator case? Is there a voltage regulator in there? If there is, is it like a mechanical switch, something that would respond to a mechanical shock?
I have no trouble changing out an alternator, but before I do, I'd like to have a reasonable amount of confidence in my diagnosis and not just go on a part swapping spree. I've also noticed, as it's been dry the last few days, this problem has vanished. No tap needed to make things happen, voltmeter at 14 and steady...
Any input would be appreciated as their isn't anything as consuming (and baffling) as an old Jeep with an electrical problem...

I would take the alternator to a local rebuilder and have it checked out. If you have a DVOM I would check voltage running. Then turn on lights and A C when idling and check voltage. I don"t remember the exact year when they changed from external to internal voltage regulator. Google a search for voltage regulator.

Well, it would seem the voltage regulator on this year is not in the alternator, so... As I can't determine there are any other "make/break" components in the alternator case proper, this is beginning to smell a lot like "Brushes!"
For $10 and $10 shipping (rip off) I guess I get to find out...

Remember when installing brushes their is a small hole in back of alternator. I insert a small gem clip in the hole this holds the brushes in place so you can install the back of the alternator. Pull it out after back of alternator is installed. I've had a wire to the brush break but it won't charge if this happens.
Ford's external voltage regulator has to be grounded I've had to use a rolac pad and clean up the metal they were mounted on or run a ground wire to the case.