[Bushleague]

For deer, lift the head and get one antler resting on the tailgate. Then try to keep it there while you climb into the truck, then grab the antlers walk backwards using your legs to drag it up into the truck. Helps if you have a truck bed liner with some grip to it. If you have two people things are easy, if your deer has no antlers stand in the truck and use a rope to drag it up. Usually there's a rope around my antlerless deer's neck by the time it gets to the truck anyways.

For moose or elk... quarter the darn thing, just about all of the most stupid things I've ever seen attempted involved trying to get a whole moose into the back of a truck. You have to cut the thing into pieces at some point anyways, just get it over with right away and save yourself the headache.