2001 GMC Yukon squeaking brakes

I am becoming a little frustrated with the squeaking in my brakes and I cannot figure what is wrong. Recently, I replaced all four brake pads and rotors in my 2001 Yukon. I purchased the parts at NAPA which I have been told by a repair shop makes the best replacement parts, versus Checker or O'Reilly for example. Anyway, they had four levels to choose from on the brake pads. In the front, I purchased the highest grade they had as it was recommended to me by the salesperson at the store since the majority of the stopping comes from the front. They are doing a good job and I have not heard anything since the repair was completed. The squeeking is coming from the rear. In the rear, I purchased the second from the bottom choice in brake pads. I have put lubricant on the backside of the pads but this has not seemed to fix the situation. I will point out that they squeek is not heard constantly, it is only heard when the brake is engaged about 25%, once the brake is mostly or fully engaged the squeek stops. I am thinking that I need to take back the pads and get a better grade. Couple more items, the pads are ceramic and I have checked the squeek sensor to see if it is bent causing the sqeak.