Ford Explorer wont crank

Next time this occurs - try starting with shifter in NEUTRAL.

When you say "battery is on" or "battery is off" - do you mean that there are no dash lights? Or there are dash lights but turning the key nothing happens? Some of these year's of Ford's had an issue with the starter solenoid signal wire. Also -- there was a service bulletin -- 94-8-14 -- which addressed multiple wiring harnesses being affected by battery acid damage and contamination. I'd have a really close look at all relative wiring harnesses.