im also with checking the distributer/ timing chain.. pull off dist. cap.. (back of engine can be a pain in the neck) and have someone just 'hit' they key.. does it turn? if so .. you need to check spark.. with oil pressure dropping like that.. and then just shutting down.. it does indeed sound like a timing chain.. and alot of time you wont get any warning.. if the timing skipped.. depending on how bad.. you might not have even noticed.. i would hazard to guess.. it is the timing chain.. or the distributer itself..

 

btw.. if the timing skipped enough.. it could cause this.. the drop of oil pressure is when it skipped.. and the truck's computer compensated to an extent.. but then shut down to avoid damage.. just because you have spark.. isnt all of the battle.. you have to have it in the right order.. hence timing chain.. more than likely it didnt break.. but it skipped.. not too un common.. how many miles on the truck?