Is walking on the treadmill at a high incline..

i'd try 5-6 as an incline. good for butt and legs.

I gotta be honest... I don't know exactly the difference incline make for leg shape because I rarely do low incline. I actually feel awkward on a treadmill at less then 3% incline, even when running.

High-incline treadmill is my go-to cardio workout, but the quickest leg changes came after doing a week of C25K at a low-incline. I could literally see the diff in a week! Unfortunately, I had to suspend C25K because of a medical setback.
My 'fat-burn' workouts (HR: 100s-120s) are usually 2.0-2.5mph at 8-10% incline. My 'fitness-level' workouts (HR: 140s-150s) are vary with intervals of the following:
• 4.0mph @ 6%
• 3.6mph @ 8%
• 3.3-3.6mph @ 10%
• 3.0-3.3mph @ 15%

I think that my calves and glutes had benefitted from high-incline, but again, I can't really say that it wouldn't have happened at lower inclines. I can state without a doubt that it has not adversely affected my knees! I used to wear 1-2 knee braces and sometime an ankle brace whenever I had to do a lot of walking (treadmill, shopping, amusement park, etc.) Nowadays, I rarely wear any braces regardless of the activity. I do keep 1 knee brace on the treadmill just-in-case, though!