Not sure what kind of truck you are talking about, but with the SUV tailgate I lean the side of the cased bass against it, then raise it like a lever with the end pin crutch to horizontal position. From there, I put my leg on the bottom bout and grab a handle on the top side to guide it into the vehicle and get the bout over the folded up seat. But the bass is actually pushed into the vehicle with the leg and hip. Then a bungee cord keeps it from tipping.
I’ll be 61 at summers end. Probably about the same age my grandma was all those years ago when she used to fuss at me to “lift with your legs, not your back” when I was just a wee lad. It was part of her “They Say” collection of admonitions, which she kept sharp by practicing them on us at every turn.