The EAT19559-010 is the correct Limited Slip differential for your 1999 Chevrolet Z71 if your truck is equipped with a 8.6" 10 bolt rear differential. please count the number of differential cover bolts to verify there are only 10. if your truck has 14 cover bolts our truck is equipped with the optional heavy duty 9.5" 14 bolt rear axle and will require the EAT19590-010. for the 8.6' use the G/235-2022 master gearing kit and with the 9.5" rear use the G/235-2010 with a Nat710507 Pinion Seal. we would like to recommend that you use a quality gear oil like the Lucus LUC10042 and a bottle of D/S43161 Friction Modifier.