If the ESN/IMEI is blacklisted or marked as "bad" or stolen by Verizon, there's not much you can do about it. You can put a T-Mobile SIM card in the Droid Ultra and see what happens but more than likely it's not going to work - I'm not 100% certain that the "blacklist" that Verizon maintains is connected with the one that T-Mobile (and other carriers) are now utilizing to track/keep stolen or reportedly lost devices, as well as those where customers just don't pay their bills so the device gets on a list too.
If you put a T-Mobile SIM in the device and it doesn't connect to their network immediately (because T-Mobile SIM cards allow that, you can access the T-Mobile website but nothing else using their 4G HSPA+ or LTE networks) then more than likely you're completely out of luck in most all respects.
All you could do at that point is contact Verizon in some manner and ask if the device was reported lost or stolen or something else. There's a good chance they may not help you at all if you're not a customer, and if you go into a Verizon store directly to check it they may just confiscate the phone completely.
How do you know for sure it's a bad ESN on Verizon? Have you checked it against something like Swappa's online checker (not 100% accurate but at least it's something):
Can't hurt to at least give it a once over and see what results you get.
Currently using nothing at all...