Thanks for the advice.
After endless phone calls and being placed on hold, someone at a truck repair facility was able to cross-reference the Fleetguard number to a Baldwin number, which was then cross-referenced to a Wix number for me. Armed with that info, I started calling Napa stores again.
Out of all the Napa stores in the Bay Area, one store had a Wix 51724, which is a 10 micron hydraulic filter. I guess it doesn't matter as I just installed a 2-micron Racor in front of it.
Cool, I get to drive my truck the rest of the weekend!