Got busy with work/life, but found some hours here and there.
There were indeed supposed to be check balls in the VB. I contacted ShiftRite and at first they were apologetic (though didn't offer any sort of refund for wasting my time troubleshooting their error), then I figured no big deal I'll just see if they have any input on my continued issue. I typically wouldn't expect free consultation from a shop for a DIY'er, but it seemed the least they could do after wasting a day of my time, however many quarts of ATF, and a good serving of frustration. Well, they thought different I guess. They stopped replying. Oh well, their business practices will catch up with them.
Anyways, I ordered Sonnax 56532-01k, a reverse servo sleave kit as a last resort before dropping the trans and digging in. Not sure if I mentioned, but the filler neck was corroded and leaked, which caused fluid to drip all down the passenger side of the trans and onto the cat-- I pulled that, cleaned it good, and jb welded it. Well, when I went to pull the pan to get at the reverse servo again I noticed there was still a leak. With the cat and the heatshield there, I couldn't really see up there. Well, I cut out the exhaust, pulled the shield, and guess what I found...
Intermediate servo cover snap ring must have corroded and failed, letting the servo cover cock to the side, pinch the o-ring and create enough of a leak to cause the gear that requires the highest pressure, reverse, to not engage or have weak engagement. Kinda crazy, I drove the truck 200+mi home like this! I was able to get the snap ring and had a close enough O-ring on the shelf. Case casting that retains the ring doesn't seem to be damaged at all. Truck drives as it should now. Gear engagements can come with a bit of a clunk, but nothing too harsh, and from what I understand this is a feature of the shift kit that came in the VB I bought. Had the dealership special order the correct o-rings-- I'm going to replace the OD servo cover snap ring too and may as well do the O-ring while at it.
So that's it. Thanks for all the help everyone. Automatic tranmissions were mystical magic boxes to me before setting out to solve this issue, and while I haven't by any means mastered them, I have a good foundation from which to troubleshoot them. That wouldn't have been possible without all the generous assistance offered in this thread. So, again, thanks!