How did you test the alternator? Did you use one of those home tester thingies, or did you take it to an auto parts store to have it tested?
I suggest doing the latter if you hadn't yet. Those home testers are great at some things, but I highly doubt it could really tell you if the alternator is your problem or not, because it sounds like it's either that, or a really bad connection. How much cotorization is on the battery posts and wires? If there is a lot of build up, it can start making some weird stuff happen, plus kill the truck.
1992 Ford Ranger XLT 4X4
Stock tires, blown engine, aesthetic accessories.
2005 Chevy Silverado 1500 ex cab 4X4 Auto
4.8 V8, No options, other than AC, Four Door
Kenwood CD/MP3/WMA/USB and Sattelite Radio Ready Head Unit, AEM Brute Force CAI with a K&N cone filter, 33X12.50/17's