The air that comes out of the pass side vents will not get as cold as the air on the drivers side vents, I have a 2001 disco II

 

I had the same problem on my 01. Just topped off the refrigerant in the a/c system and its been good as new.

 

The freon level has no bearing on the balance of cold air coming from the vents. When one side is warmer then the other there is a problem and it should be checked.
You might read up in the owners manual on how to perform an A/C diag. test, run it and get back to us with the codes. When you do the test, make sure you are outside, in the sun light and the A/C has been running for a couple of minutes.
Mike

 

You got it! Low charge causes this.