What can I do and do I have a case?

Sounds like fraud to me. Unfortunately, since the vehicle is gone, you do not have a legitimate legal case (in my opinion). Many third-party extended warranty companies use contractual language and disclaimers to avoid honoring claims. Assuming you have two certified mechanics who would have testified the transmission could not be open, my feeling is the warranty company acted unreasonably. (Of course I would have needed to review the warranty contract, purchase documents, etc. to know for certain). But since sold the SUV, you really don't have the necessary evidence to demonstrate the defective nature of the vehicle in court. As always, you should seek a second legal opinion, but in my view, this would not hold up in court.

My answer to whether you have a case: maybe.

I agree with the above answer that you have an evidence problem with proving your damages. Since you no longer have access to the SUV, it will be difficult for you to show that the transmission could not be opened. You could try to present witness testimony from the two shops, but that might be an uphill climb.

However, depending on the language of the contract with the warranty company you may have an action to bring under the Michigan Consumer Protection Act (MCPA), or a fraud or breach of warranty claim. At the very least, you can file a complaint against the warranty company to the Michigan Attorney General.

Best wishes,
Andrew