2006 Ranger will have "Smart Charging", this can make the battery last longer but it MUST BE a Silver Calcium battery, not the normal lead acid car battery, smart charger will ruin lead acid battery.
You also should NOT jump start this system, you can disconnect the battery and charge it, just don't do it with battery connected.
So at 9 year old is the battery factory original or has it been replaced, and was it replaced with a Silver Calcium battery?
I think it was replaced.
If battery has 12.6volts it is probably fairly new, I would guess 2 maybe 3 years old.
And I would also guess the problem is the starter motor, not the battery.
The fact that is was just driven, so charged up and after trying to start engine it still shows 12.6v reads like it has plenty of juice, and the fact you added more juice with battery pack and the starter slowly moved means to me that the starter is drawing too many amps.
It could just be a corroded cable on the starter motor end, I assume the battery end looks fine, but more likely problem at 9 years would be starter motor failing.