I think Musiclovingmom's logical explanation was really good. However, it sounds as though you've probably already tried explaining, but to no avail.
Obviously, the bald facts do make your daughter sound very ungrateful. But you say that she doesn't have a history of being materialistic. So I think there's much more to the apparent ingratitude than meets the eye.
Namely peer pressure and perhaps the fear of losing face. Yes, it's easy for us, as mature adults, to say losing face doesn't matter. But to teenagers, social pressure can be very cruel and losing face devastating. So, for what it's worth, here's my theory....
I had a minivan, loved it and still miss it. But I gather than teens don't consider minivans cool. I think that when she started driving the minivan, your daughter probably took a bit of flak from the nastier element of the rich friends for driving one. And, naively, instead of making some confident, jokey retort, she was defensive. So instead of stopping right there, the nasty comments continued and escalated escalated. basically, she and her minivan became the target of a degree of bullying. [Grrrr...... ]Hence her complaints and apparent dissatisfaction with it. At eighteen, she wasn't going to tell you what was going on. So she vented her unhappiness to you via rude remarks about the van. [If this is what happened, then I am so sorry, Heforgotme. Bullying is soul-destroying - not jsut for the actual targets. :-(( ]
When your daughter got the cheque, she was excited and she may well have told people that she planned to buy a car. But then she found out how much cars cost. And reality set in. She made the better, more logical choice. (Yay!) But now she is upset about losing face. And THAT is why she is behaving so badly.
At the moment, like many a teenager, she is totally focused on her own feelings. In this situation, I think she would benefit a lot from a shift of focus and being encouraged to think about YOUR feelings.
So I'd start by asking her non-confrontationally, in a quiet moment, whether she realizes how hurtful it is for her father and you to hear her talk in this way. I'd remind her how you've always supported her and tried to do your best for her. Of how until now you've always been really proud of her and of her excellent values, etc. etc. And since the two of you love her so much, you were really excited about being able to give her what to you and her hard-working father was a very generous gift for graduation.
But now, instead, the two of you are both upset and really hurt. Because she has somehow changed, from your loving, proud-making daughter, to an apparently ungrateful and selfish brat, who is not only saying that your carefully budgeted-for gift is not enough, but is even accusing you of taking back the cheque that SHE chose to exchange for the minivan.
I'd pause. (Perhaps to wipe awya the tears that I'm sure by now would be streaming down my face.) And then I'd ask quietly, "Daughter, this is NOT the real you, is it? So what's really going on? What's behind it all?"
If it didn't all immediately come pouring out, I think I'd perhaps ask outright whether it was something to do with the rich classmates, who had more money than character. Had some of them made immature and unpleasant comments about Dad's minivan? etc. etc. And then wait. And maybe give her a hug. And hope that I found out the truth.