I use a black grease on the ring/pinion gears- good coverage but sparingly as well as the diff outdrives where they mate with the diff cups.
I am experimenting with graphite lube or a dry bicycle chain lube on the axle universals.
Shock o-rings I use Slick Honey (Slickolium) but will be trying some green slime grease next time I overhaul shocks.
I don't think there are any greases that will harm the plastics as long as you stay with regular bearing grease and not some oddball grease specific for some other application.
Black grease that comes with the kit for the ring gears is good stuff.