I have no trouble engaging the butt and hamstring muscles.
Elk- I just sit up straight (like you can do now with your albatross bars!) and concentr4ate on relaxing the front thigh "quad" muscles.....instead, try to power your pedaling from the butt and hip joints and keep relaxing the tops of your thoighs. I find this works best on a long low incline where it's good to switch to after your quads are burning for a while. It's almost a mind thing- your muscles follow if your mind keeps telling them.
I learned that "power up from the hips and butt" trick right here on TE under a hills thread. It's GREAT when you start running out of steam on a long hill.
....But it also happens to work your butt and hamstring muscles nicely.

And yes, standing and powering your way up a hill will help those muscles too.
But don't forget you WANT bigger quads anyway- they are the biggest most powerful leg muscle i think.