This is my first truck and its an 87 f150. The rear tires will not come off after i took off the lug nuts. There appears to be a double half-moon shaped mechanism in the center of the wheel and dont know if that is a lock of some kind requiring a key. Anyone have any thoughts on this?

 

Can you post a pic ?

 

im thinking there is a tool for this

 

The tire is probably just seized to the drum, try kicking it towards the bed in different spots. That is part of the axle and the wheel slide around it, it doesn't "lock" anything.

 

This. It happens with disc brakes as well. If kicking it doesn't work, get a hammer and lightly tap the rim around that ring in the middle. It should break loose.

 

They're rusted on. If kicking doesn't work, you might have to get out a sledgehammer. Don't hit the rim with it though, just the tire.
I usually put anti-seize compound on the contact surface before putting it back on.
Where the wheel touches the drum. And If I have the drums off I put some where the drum contacts the axle plate, especially where the circle in the center fits over the axle.

 

You can also put the lug nuts back on, but don't tighten them down. Either grab the side of the bed and rock it back and forth a bunch of times or get in it and drive slowly around your parking area to help break it loose.

 

Happened to mine.... it took a Bfh ( big fn hammer ) and some break or carb cleaner on it not sure which one.. To break it loose.

 

Yeah I've had one stuck now & then on older cars
Once I get it off I sand down the rust to make it easier to put back on

 

My mother used to keep my little league bat in the car for when she changed her tires, one hit to either side of the tire and it would fall off. To prevent this all you have to do is rotate your tires with some regularity.