The two best ways to get more ass, muscles involved that is, is first to really squeze the glute/hamstring tie-ins at the top of a supersetted DB SLDL and a BB SLDL.
Also explore new and painful (hard exercises), like my second recommendation.
Place a bench behind a smith machine, paralell to the bar, approx 18-24 inches awy depending on your height (Kurgan may need more space!) Place one foot (the front of the instep) up on the bench, and the the other leg foward of the bar so that when your rear leg's knee barely touches the floor, your front knee is at a 90 degree angle (knee not going out over your foot). Do one leg, then the other for 8-12 reps, times 4-5 sets, raising the weight each set. Kind of looks like a cross between a one legged squat and a lunge!
This will toast your butt, but you will not feel it there during the exercise!
As for your other exercises, I would recommend a total change next W/O and then never do the same W/O twice. Add in some front squats, hack squats, DB squats, high rep work, especially with extensions and leg curls, and really push the envelope!
Josh, there is part of the problem right there. Try more sets of squats, pyramiding weight up and reps downs, and then down sets with higher reps once you are exhausted!
12 X warm up
8 X weight 1
6 X weight 2
4 X weight 3
8 X weight 4 This is the prime work set
15 X weight 3
20 X weight 2
25 X weight 1
or if you don't want down sets, then do 3-4 sets at your heaviest weight after 2-3 w/u's 8,6,6,4
When I talk about high rep, like leg extension I mean 40, 50, 60, 80 reps
As for step ups, I spend more energy on balance than on the weight, so I guess I'm a step-up wuss!