3rd new starter in 6 months

I had my original starter go out (117k on 1992 trooper) and had it replaced by a local shop with a NAPA starter. Luckily guaranteed parts and labor one year as the first one would stay engaged a second or two after it started so it made a grinidng noise. They replaced that and then this week I started up in the morning, drove 20 miles, started back up in about 20 minutes, drove another 20 miles, then when I went to start it acted like the battery was almost dead. (Battery is an 2006 - 84 month Autolite Battery) It did start but barely drove about 5 miles and stopped to get gas and it was stone dead. Would not crank at all. Jumped it with a set of really good jumper cables and a diesel truck that put out a lot of juice and after 15 minutes still nothing. Got towed and the shop replaced the starter again, it was bad, but no explanation. Before I get stranded in the toolies any ideas as to what is going on? Battery checked out ok.