I have heard this many times :)
I'v payed 100's to get that same answer from physio's. I figured out that it was my sway back posture that was causing most of my problems.
to be fair, there's a little more to it than simply strengthening glutes, but yeah, your sway back posture is an indicator that something's wrong.
And it's a nightmare to fix. Chances are you'll never fix it, but you can certainly improve it.
Get in the gym and get your glutes stronger. Doing things like glute activation work and core stuff at home on a daily basis will help but you need to get in the gym and strengthen the movement patterns.
every time you hit the gym do a few sets of bird dogs, glute bridges and side lying clams (youtube has descriptions of all of these) as part of your warm up. That should loosen you up a bit and give you the mobility you need to train efficiently.
be patient, it'll take time but you'll get there eventually.