"I have never dealt with this before and I don't know where to look for my information."
You start by calling the lender. You have either made your payments on time and in full or you haven't.
"Can they legally come snatch my car from the store like that at midnight and leave me stranded?"
If you haven't made the payments, Yes.
"Is that legal and if so how?"
Apparently in OK it is legal. There are few rules in OK regarding reposessing vehicles.
"Can they charge me to give me my personal items back (which, by the way, they took to OKC, over an hours drive from me)?"
Their lot and company, their rules. The lot is where it is. What ever company the lender contracted with they are not obligated to be close to you.