What brand of shocks should I buy for my 03 tahoe?and do I need to replace all at the same time or only then one that is bad?

i would always change them in pairs, both fronts or both rears. Just depends on what kind of ride you want, realize the more you spend the better the ride. Cheapie ones are just that, and high end ones perform better. You have to decide on that one. DO NOT buy the chinese made cheap ones if you care about ride