Help reverse gear shat itself

[Vortex-98]

Hi guys having this problem with my AU1 5.0 an not sure if its diff or trans is to blame....so the other day i parked on a steep hill,which was silly but I t s worked every other time on this hill ,so didn't think I'd get stuck an lose reverse power ..weeks before the diff would make a noise like the wheels jump or spin for a second or something if it had extra load on it,on level ground was fine so that's what's happened it's loaded up to reverse up the hill an something's let go an I've lost all drive in reverse all others are fine ...would it be a diff problem or a tranny problem...
An yes I've added a litre of ATF hoping it might be just low but nothing has changed