Why is your truck in park while waiting for a light? Allowing the weight of the vehicle to be held by Park without any brake is very bad idea. The proper use of Park is to first engage the Parking Brake and then put the gear selector into Park. Allowing the weight of the vehicle to rest on the Park "gear" is a very good way to damage the transmission.
This is just wrong.
Always put it in park for an auto, and 1st gear for manual. Let the car settle before you put the parking brake on.
#1. In an automatic transmission, when you place it in park, a pin is what actually stops any forward or backward movement. No gears what so ever are engaged. In this scenario. The parking brake is there as a fail safe encase the pin breaks.
#2. In a manual transmission, you let the clutch out and engage the gears to make sure the motor is holding the car when the engine is off. Here, yes the gear is doing the engaged, but the parking brake here is doing more of the work. Many cars have a low enough compression that a car parked on a hill can creep because the engine wont hold it.
In either case, if you put the parking brake on before you put it in park, or 1st gear with the car off, you run the risk of the car rolling away if the brake fails. There will be built in slack in the drive line and could cause the pin to break in an auto, or to pop out of gear in a manual.