Hi Irish,
Sorry you're dealing with this too. Back pain, hip pain has got to be the worst kind there is.
It was a bone scan that diagnosed my sacroiliitis. Not a bone density. Two very different tests. If you had a bone scan done and an MRI, those are the best diagnostic tools for you.
Yeah, an xray will only show if there are any fractures. The bone scan shows more than just bone.
I'm still waiting to see if my sacroiliitis is more than just that. The question of Ankylosing Spondylitis is still going around, but my rheumy said AS and lupus don't present themselves together. Whatever.... 
You should get a referal to a rheumatologist. They specialize in arthritis and rheumatic diseases. Much better to see them than your GI doc. They can take a few months to get in to see, so scheduling an appointment should be done sooner than later.
I hope you get the answers you need really soon. It's not fun living with this pain is it.
Blessings,
Ginny