strap the deer to a ladder, prop one end onto the tailgate, then lift and slide the deer in.

or

I had a game cart similar to the looks of the cabelas super mag hauler, and modified it. I made sure the wheels were easy to remove, and also modified it so it could be locked in a flat position, then loaded it like the ladder, in the example above.

or, you might look into the gutless method, and quarter your deer in the field. there are a few dvds you can buy, or I found this video to give you a general idea:
http://elk101.com/webisodes/gutless-video/