The factory Z71 wheels are 15x7 so 15x8 will just spread the sidewall a little wider than what you have right now. Should be fine. I'm running what is essentially a 33x12.50 tire on a 16x8 wheel. It's not too narrow, but is on the narrow side of the range. Keeps the rim inside the plane of the sidewall and protects the rims offroad.
2007 TOTW #33 - LIFTED CLUB #24 - CONSERVATIVE CLUB #43
Rancho 4" / Skyjacker Softride / PA 3" - ProComp Xterrain 305/70R16 - 16x8 Eagle 1378
Smittybilt XRC-8 Winch - Optima Red - Hella 4000 Compact Fogs - AEM / PCMForLess - E-fan - Hard Cover