Thats a really hard one to comment on - you've had a Wikipedia reference-linkMRI of the back so that does generally rule that out... however I wouldn't rule out the back as a cause of symptoms - your symptoms seem to peripheralise and centralise which is very characteristic of a discogenic problem.
I'm not sure about your gluteal tears - I'd look into an ultrasound of the area to rule those things out.
Maybe someone else will have something a bit better than that... but I think you need to bully your doctors into more scans. even MRI's won't give you the right answer first time and they find things on repeat. I'll read it again later and see if i can think of anything else but my patient just showed up so i better go.