Thank you for the additional information.

Xanax is used in dogs sometimes for separation anxiety but is not likely to help in this case. She may have injured the soft tissue in that back leg or possibly torn a ligament. The best thing you can do for her tonight is cage rest her in order to allow her to rest that leg. Pain control is also a good idea. An over the counter pain medication that is safe for dogs is Buffered Aspirin. You can give 5 mg per pound of body weight every 12-24 hours. For an 80lb dog you can give 325 mg tablet once or twice daily. Just make sure you tell your veterinarian that you have given her Aspirin. This medication cannot be given with other non-steroidal anti-inflammatory drugs. You can also ice the leg a few times tonight for 10 minutes at a time.

To help her sleep tonight you can giver some Benadryl (diphenhydramine), 1-2 mg per pound of body weight every 8-12 hours. So you can start with 75 mg and move up from there. 75 mg should be enough to take the edge off. Again, let your vet know if you have given this drug. Benadryl will be safer than Xanax.

Your veterinarian will likely sedate her tomorrow and take some x-rays of the leg and manipulate the leg to check for a ligament tear.

Good luck. Please let me know if you have any other questions or concerns.
Dr. May