A guy taking a nap in the back of a vehicle.
A man is dozing off while bumming a ride in the back of a truck.
The man is not necessarily bumming a ride and the vehicle may not be a truck.
Being in the back of a vehicle does not imply one is bumming a ride.  Not all vehicles are trucks.
There are many types of vehicle other than a truck and a man riding a vehicle may not be bumming a ride and dozing off is the act of going to sleep while taking a nap means one is already in one of the sleep states and a guy can be a boy or a man when used in the singular.