I need some advice on our 4-year-old boxer dog.  About 3 weeks ago we noticed that our guy (Cash) was limping on his back right leg.  We took him to the vet who said it was some sort of traumatic incident, but he was unsure of the problem.  The dog had only ever been inside, in the yard with his buddy (another boxer, 9mos.) or on a leash for his walks.  We have no idea what could have caused the trauma.  He was put on cage rest and aspirin, and we noticed him getting better this past weekend.  Today I noticed that he seems to be carrying his left leg up higher now and seems to be in even more pain.  When he's lying down, he whimpers when I press in between the bone that sticks up on his back end and his spine.  Not sure what this is?  Pelvic injury?  Tendons or Ligaments?  We know it's not his ACL or his hip since he has full range of motion in all joints.

Anyone have any ideas?

> Anyone have any ideas?

Take him to the vet. If you did take him to the vet before (I find it very strange that a vet told you to give him aspirin), I would take him to a different one. I would have expected the vet to prescribe an anti inflammatory and then to have x rayed him by now.

It could be any number of things, it's impossible for someone to diagnose your dog over the internet.

I dont think you can entirely discount cruciate at this stage. It may not have ruptured  (therefore giving the usual signs) but it may have partially torn and needs time or surgery to recover.
Boxers, because of their very defined thigh muscles and rough play, are very susceptable to this injury.

Could be a trauma injury to his spine. You mention that you have a young boxer. Well I know of a dog that had a trauma injury to the spine from one of his other dogs landing on her. I am sure they probably play quite rough together and boxers are very bouncy! Your Vet should be able to tell were the tenderness is.