You're still initiating the descent with your hips ahead of your knees and touching the TUBOW later in the descent than you should be. It's possible that you're squatting with your knees too forward and your torso too vertical the rest of the time. Feeling these in your back may mean you're actually creating the conditions to shift as much load as possible onto your hips (since this creates a more horizontal back angle), a feature as opposed to a bug of the squat model advocated here. Keep working on breaking at the knees and hips together. Touch the TUBOW ASAP.