You must have had that noise before the idler arm replacement, to find what you described. the GT4 is the RPO code for the 3.73 gear but G80 would be the limited slip diff. The G65 is not one I am familiar. The G66 and G67 have to do with auto and manual leveling.
G65 is the RPO code for the rear manual self-leveling Nivomat shocks, if they're still on the truck. Part of the Premium Smooth Ride option.