My 1993 F150 5.0 EFI Auto. has a fuel vapor canister leak, have been smelling gas when I run the air con. and I just noticed today that it was dripping onto the driveway. What can cause this problem?

 

The dripping on the drive is normal that is what it is suppose to do.
You do not have an air intake under the hood where the fuel vapor canister is. The air intake for the A/C system if on max is the passenger compartment and if on normal is in front of the windshield.

What you smell is water that has dryer on the evaporator from not switching your A/C system to vent about 5 minutes before you shut the truck off.

I do not think that your vapor canister is leaking unless you removed the hose going to the throttle body.

 

SubFord, it is dripping Gasoline from the vapor Canister (mounted on the frame pass. side right under the battery).

 

Then it sounds like you have a bad vapor valve on top of the one of the fuel tanks.
Or your solenoid valve is never opening or the hoses are no longer connected to the throttle body.