Is your Ranger a 4 cyl, or 6? The only wire I see different between the '99 V6 Ranger and '99 V8 Expo at the 42 pin engine/body harness connector is a VSS and PCM ground at pin 13, and I suspect that would only apply to a '98 Expo, not your '99/'99 mix.
If your Ranger is a 4 cyl, you'll have to deal with PATS since the Expo PCM won't light off until PATS is happy.
If a 6 cyl, I think even PATS will be a non-issue. Both '99's use the same, external to the PCM PATS system so I think the Expo PCM will light off just fine with the Ranger PATS giving the go-ahead.