Why could all my brakes be locking?

What is your question?

Just in the past week I have noticed my 2004 Yukon Denali brake pedal going hard and my brakes locking. Forcing the truck on the road as that's when it happens. The Service Stability as come on when the happens. Any body have any clue? Thanks in advance

What makes this problem better or worse?

When I brake

How long has your 2003 GMC Yukon XL 1500 had this problem?

One week, two tops