I don't want to start another rant about Autozone--might get me stoned with rotten tomatoes.
A starting battery has plates with lots of area exposed to the electrolyte. The plates are good for lots of amps, briefly. But they break down under deep cycling--meaning lots current drawn out of them. They should not be drawn down to under 90% capacity.
A deep-cycle battery has heavy duty plates that thusly have less area exposed to the electrolyte. They can't put out as much current, but can be drawn down to 50% regularly without damage. So if you have had batteries for 8 years and been winching on them, they could be building up damage. I have had an old battery that would immediately smoke when I hooked just a 10-amp charger to it.
I had another battery buried under the seat in an RV. I left the RV plugged in all the time, and the converter had a 12v charger. Over the years the battery had boiled all of the electrolyte out. One day as I was running the electric jack to unhook it from my truck, the battery exploded. There was no flame, just a bang that moved the curtains and brought half the campground running. Fortunately it was before 9-11.
Yeah, an 8-year old battery can just die. That's a long life for a starting battery running a winch. If your alternator is checking out fine, which it seems to be, I would install Wal-Mart yellow tops and move on. I take mine in every 3 years and get a new one for $20 whether it's bad or not. The first one died in 3 so I figure they always will.