Once again, Melanie, you've saved my bacon!

It was Vref+ and Vref-. This is what I needed to do:

ADCON1 = %11101000

Bit 2 "0" selects Vss (internal) for Vref-
Bits 1-0 "0" selects Vdd (internal) for Vref+

No wonder the result was screwy!

Thanks!

Tim, thanks for your note, too. I'll remember that as a troubleshooting tip for the future.