You can, but if you're not quick enough, the truck can move when you set it to reverse. I hope that the truck is not on a slanted hill either; that would ask for more trouble.

When you unlock the steering mechanism via the ignition switch, you can select any gear that you choose.

Usually. Some vehicles have interlocks preventing the shifter from moving with the ignition in the "Off" position. Others only unlock the shifter when the brake pedal is depressed to the floor. Yet others only require that you move the shifter a certain way; I had one that had to have the shifter tilted left about three inches before you could pull it backwards.

There seems to be three slightly different answers here. Let me clear it up. Some automatic transmissions have interlocks that keep you from shifting out of park without either a) putting your foot on the brake or b) turning the ignition to the ON position. jerv is almost correct in saying 'usually'. The correct answer is always. Even with the interlock, you can simply put the brake on, put the key in, turn it to ON (you don't need to start the car), and shift as you wish. I do this when I'm pushing my car a few feet here and there where it's not worth the gas to start it. Kyogre says the vehicle may roll, but you should never shift without having the brake on anyways, so that shouldn't be a problem in the first place. I will caution you, however, that when the vehicle is off there is no: a) power steering b) power brakes c) gauge readings, etc. So, if you are going to put the vehicle in neutral, you will have limited control over the brakes and steering. The steering will be very tough, and the brakes will require a LOT of pressure to stop the vehicle. You usually get one push of the brake pedal (this is the boost still remaining from when you last had the car on) but after that the pedal will get very tough. Hope this helps, Good luck

One thing to be aware of is that you should not tow any modern vehicle with automatic transmission for a significant distance (many miles) with its drive wheels on the ground unless it is a 4WD with transfer case that can be put in N (which disconnects driveshafts from transmission). Older vehicles lubricated the auto transmission from a pump on the driveshaft end, but newer auto transmissions do that from engine side. So with the engine not running the transmission may not get proper lubrication from just the drive wheels turning.