Have a 2012 gmc 1500 4x4 SLT CREW CAB would like to put a lift on it any recommendation on how high 2 or 4 inch and price will it affect my warranty

2 inch is pointless its just a leveling kit, 4 is roughly the same price and will actually lift the truck, as for warrenty the dealer has to prove it was the modified part that caused the warrenty issue, ie if u throw O2 sensor codes the can not contribute an O2 sensor being defective as to the lift. if u end up having a loose ball joint or tie rod ends they could be contibuted to a lifted truck and dealer may not cover them under warrenty

I agree with Chris. Go with a 4" lift. Throw on some 33's and you will be pleased. Just make sure you do a suspension lift and not a body lift. Body lifts are only useful on top of a maxed out suspension lift when you need to fit larger tires. For a 4" lift, you would get a kit with 4" spindles and 4" rear blocks. This would level the truck. However for about $100-200 more, you can get a spacer for the front struts and raise it another 2". You will then want to add a leaf spring to raise the rear end more to keep the truck leveled. I lifted my Nissan Titan with a 6.5" lift in the front and 4" rear blocks in the rear and that leveled my truck perfectly and threw on some nice 35" tires.