you sound just like me! i have been having the same kinda problems for the past year.... and as it turns out, i've now replaced both the alternator and the battery (which fixed all problems)....
but that doesnt help you, eh?
my best piece of advice would be to take it to a shop which can test your battery and alternator in one shot.... i happen to have this done at Sears, but a lot of shops can do it for cheap or even free --- they can hook a machine up to your battery (while the truck is running), and test for battery power, and charging power (from the alternator)..... that way, you can pinpoint the exact problem....
that's my suggestion....
anyone got any others?
good luck!
jewels.