Have you checked your serpentine belt? There are a few discussions on the forum about how easy it is to incorrectly route the belt when changing it. Also if it's old it could be stretched out, losing tension. Either of which, in theory, could cause your alternator pulley to slip and run slow, thus not operating to its full potential.

 

I just replaced the alt and the truck still dies right after a jump. The belt is good and spinning the alt just fine. I'm stuck...I guess I'll check the connections...pita...

 

If it keeps running once jumped with the cables on, then dies when you remove them, does sound like alternator, cables, or that 100 amp fuselink in the underhood box. The front two cables in the underhood box bring the alternator over to the rest of the truck.