It's running again!
Sorry for the hiatus. It's just taken me this long to actually try what was suggested. I borrowed a trickle charger from a friend and charged up the battery for a few hours, then put in 1 gallon of new gas (the tank was on empty), and cranked it, and it was the same as before: it would start but would die. I did it again and kept my foot on the pedal and ran it for 5 minutes. Still died. Then I had to run another errand, so I got another gallon of fresh gas (thinking that since I only put in a gallon of new gas, maybe the new gas just didn't get into fuel line). So I put in the second gallon, cranked it, it died, cranked it again and kept the pedal down for 2-3 minutes, and it idled by itself.
Thanks to dhvaughan for his advice, as well as to the rest of you for taking the time to respond. I've got other (mostly cosmetic) issues with this truck, so I'll be posting more questions....