Thanks, guys, for the all the help. I appreciate it.
I didn't trust my late night trace inspection last night, so I went over the chassis again, wiggling all pins all the way to the yoke pins. When I got to the yoke pins, I got nervous. I pulled out the meter and tested continuity from the pins through to the board. One pin was good, the other was not. BINGO.
I HAD re-soldered the pins, but obviously there was old solder beneath, because my added solder did not fix the joints. I sucked all of the solder off, resoldered all 4 pins and was greeted with this on my tube:
Well, at least I fixed the deflection problem anyway. But alas, this tube's yoke is DEFINITELY for a medium resolution chassis. So, I'll have to pull this yoke and install one that's compatible with this chassis.
For kicks, I took the chassis downstairs to my game room and dropped it into my Ms Pac, which has a 4900 in it too. Unfortunately, my Ms Pac is using an early version 4900 (023) board, so the yoke is an early version yoke. I got foldover on the image that can't be adjusted out. But the part of the image that I COULD see was freakin' BEAU-TI-FUL.
FYI - BIG thank you to Peale for putting this info out there, it was very helpful:
http://forums.arcade-museum.com/showthread.php?t=142056
Without actually SEEING a perfect image, I'm pretty sure this chassis is working just fine. I have one more 4900 in the Neo Geo on location at the pizzeria, so I'll go have a look at the board in that one, and if it's a match to this chassis, maybe I'll test the chassis in there to make sure everyting is A-OK, but I'm pretty sure this chassis is good to go.
So, long story short: cold solder joints on the yoke connector pins for the vertical.