Okay, I think I fixed my issue.
After upgrading my ROM, I was having trouble getting an EVDO connection, which I now think was caused by just being in an area where there is little or no EVDO service. I read on several posts here and on xda where people were having similar sounding problems that the HDR AN AUTH password should be your hex ESN, so, upon going to ##778#->security and seeing a bunch of garbage ascii charaters in the password field, I assumed this was my problem. So rather than being cautious and doing a little more research, I replaced the password with my hex ESN. This was a bad move, because if I needed that ascii garbage password, there was no way to get it back.
Luckily, after much searching, I found what I think is the exact same garbage ascii string in ##778#->M.IP Default Profile->AAA Shared Secret. I was able to copy that string and paste it into Security->HDR AN AUTH Password. Now everything seems to be working great.
So just to summarize, for anyone else who might be wondering if they have the correct settings under ##778#->Security, here is what is working for me:
HDR AN AUTH User Id:
HEX_ESN@hcm.sprintpcs.com
HDR AN AUTH Password: ascii garbage (a lot of "□"s)
PPP USER ID: blank
PPP PASSWORD: blank