To be shot in two years, sounds like you've got a short in a circuit that is causing a high amp draw when you leave it parked. Try hooking up an ammeter between the positive battery terminal and the positive cable (been a long time since I've done this, hopefully someone can chime in if I've misspoken on placement). A google search should verify placement.
Once hooked up properly, check the amount of amps being drawn. With engine off, should be practically zero. Enough to keep the clock running is all I can think of. Once you have a baseline, start pulling fuses one by one and watch the ammeter. You get a big dip in amps, you have identified the circuit.
The battery's test good. Alternator good. Tester show normal draw. I am going to unhook the battery's tonite and let it set for a couple days and rehook and see what I got.
Had a under hood light stay on a while back. Couldn't figure why my batts kept going down -- Till one night when I accidently found the culprit...
Go out to the truck well after dark and have a look around - Ya never know what you might see.