I have a 2000 silverado with a 4/6 drop on it. I put 20" rims on it and the back tires rub with the littlest weight in the bed. I am going to put air shocks in the back to replace the stock shock. Does anyone know what the compressed length of the shock is?

 

with that big of a drop you should be looking at cutting the middle of the wheel wells out or tubing them for the right clearance