Dog - when out running with my staffie he skips with both rear legs! is this normal ?

It could be an undiagnosed joint condition causing the odd gait, if he ever shows an sign of pain or discomfort or you are just worried about the odd behaviour always consult a vet. Even just call them up and do a free over the phone consult to put your mind at ease. Without seeing the way he runs myself I can't judge if it's just an odd habit or a health concern. I'm a bit biased since my dog genetically has bung knees and hip dysplasia that all will require surgery and he's not even 2 yet. You could say I'm overly cautious of any strange changes to how a dog normally moves or behaves.