^ This. Rear suspension is a big help.

Most likely the fit is just fine. Mtn biking works your lower back more than road biking. No big deal for short rides, but the aches start after a couple hours I'll bet. I did a 24 hour race and was feeling fine the first 6 hours or so. After that the pain set in. I was in absolute misery the last lap. It's just fatigue.

Solution: MUCH more core and lower back strengthening.