The main difference between the Z71 and the Z85 is the type of shocks on the suspension system. Both packages contain monotube front shocks.
The Z71 is designed for better off-road capabilities. It contains monotube rear shocks specifically placed to prevent damage on rough and rocky terrains. The Z85 also contains monotube rear shocks, but it is built to offer enhanced towing abilities for owners who need to haul trailers and other recreational items on a frequent basis. The Z85 is not designed for off-road performance so it offers a smoother ride. The Z83 differs from both of these types by containing twin-tube rear shocks.