Look in a junk yard, see if you can't find a wrecked truck with a good motor still. Listen to it run, let the engine warm up, make sure there isn't anything funny in the oil, like coolant or an additive. Just check classifieds, sites like these in the for sale section, gotta look everywhere till you find a deal that suits you. Out of production since '92, 4.0L is a good donor motor. 5.0L swap isn't that
bad, but definitely takes some time to do a decent job.