There are several ways. Helpers would help, but you and the driver can manage if you're careful. Here's one idea. Someone will be along in a minute with a better one:
Use a pry bar or something to lift the crate off of the floor of the truck.
Shove 2" pipes under the crate every 2' to act like a conveyor so you can roll the crate off the truck.
Roll the crate to the end of the truck so a foot or two protrudes beyond the end of the truck.
Place a 6"x6" beam under the crate so 6" extends beyond each side. Put an eyebolt or strap through or around each end of the beam.
Use 2 engine hoists to lift that end of the crate using the beam/eyebolts/straps. Have the driver roll forward with the truck until only a quarter or so of the length of the crate is left inside the truck. Lower the suspended end of the crate to the ground.
Remove the 6x6, move it to the other end of the crate, lift that end of the crate using the engine lifts, then move the truck out of the way. Fully lower the crate.
Think through each step clearly. Uneven loads, unlevel surfaces, etc. will make it more challenging.