Shock question. Please point me in the right direction

I have a 2008 Duramax and I thought my shocks were still serviceable until a section of bad interstate had it doing the porpoise dance. I replaced with a known brand that were decent for a short period of time, then back to the bouncing act. I finally went with a set of 'Bilstein 4600's' a year ago and my vehicle is now bounce free.