Just a quick update in case anyone else runs into this problem. I spoke with DP Tuner tech support and was told this was very likely to be the switch, but unfortunately they didn't have any more replacements left. I was contemplating getting a new modern chip when I figured, what the hell I'll try fixing the switch. First I sprayed a bunch of contact cleaner in it and worked the switch a bunch and it did help, but not completely. That did verify it was the switch though, so I carefully disassembled the switch taking care not to break the little plastic locking tabs. I could then see the oxidation on the contacts, so I used some really fine grit sandpaper to clean up all of the contacts, reassembled it and it's been working like new ever since.
'02 F350 XLT X-Cab 7.3L/Auto 4x4 Short-Bed SRW
Autometer Boost/EGT/Tranny Gauges
Mods: TTM, Ride-rites, Prepump, Poon'd, Zoodad'd
Sonnax/Tricumilator, 4" Magnaflow 3.5" dp, AIS
DP tuner (stock,60,80-mileage,120)
6.0 Tranny Cooler, EBV Engine brake