96 Yukon won't start! Starter? Crankshaft position sensor?

[BigRedYuk]

So I've got a 96 GMC YukonSLT with 180 on it. A few weeks ago my trans went out so it hasn't been getting driven much since, but I still would star it and let it run almost every day and could take it down the street to get gas if need be. A few days ago it took me about 10 minutes to get her started with a jump, an yesterday my starter died. I replaced it with an AC delco starter which provided crank but the truck wouldn't turn over. I proceeded to swap out my fuel filter an fuel pump relay (don't know how, i Hadn't looked in my main fuse box in a while, but there was no fuel pump relay at all when I checked.. could've been like that since I bought it last year). After those changes I could hear that it wanted to start more than before but still wouldn't. Eventually the starter that I just bought just stopped engaging into the flex plate. It's a high pitched whine now instead of crank. So I removed the starter and have opted for a better quality one but I am weary of putting it on if I'm burning starters out because if another issu. I've got 3 batteries in my truck total all resting at 12.4... I'm pretty lost at this point and need help! Am I missing something? Is it the crankshaft position sensor and I've been fixing the wrong thing?