Not sure where your diagnosis came from, but most of us have an anterior pelvic tilt. And most middle aged men have a pretty substantial one at that. Honestly, unless you have a massive structural deformation of your pelvis I'm not at all sure how that would affect how a pack fits you or would cause you discomfort.
The area you are describing *could* be the gluteus medius muscles, which most certainly are way under used for most of us and frankly, seem to be at the root of many, many back and lower extremity ills. Of course, it could also be a referred pain from the last lumbar segment where it meets your pelvis...also a common place for people to have issues.