Pulsating when braking between 25 to 15 MPH in 2006 GMC Envoy. What would cause this?

[Christopher]

It seems to only do this after I have been driving for a while and the engine and brakes have heated up. It only does it when I am braking and have it in gear (if I put it in neutral before braking it doesnt pulsate). It is always between 25 and 15 MPH. Took it to the shop yesterday and they said it was the rotors, they machined them, but it still pulsates. Could this be the ABS kicking in when it shouldnt be or something else? There are no check engine or ABS lights on.
I should note that when the shop machined the rotors they only did the front rotors. Could it possibly be the back rotors causing the problem?

 

[kelly_f_1999]

well if abs working you should feel it do that some but possible vaccum leak on brake booster but id say its normal

 

[section hand]

Turning Rotors is not Good Practice on the Front Wheel Drive cars,that have A.B.S. and if You do the Machine must have Cutting tool that will do both side at the same time,I Replace Rotors when I do Brake Job;

What You describe is the A.B.S. is trying to Compensate for uneven Rotors;
You are correct with Your Statement;