I agree with dattodude that the diff should be centered. However it's common for them to be around 10mm one way or the other, which is not really noticeable until you fill the gaurds out with wide wheels.

1200GXman, you say the drivers side rear is sitting about 1 cm outside the gaurd - how far in does the other side sit relative to the gaurd? If it is 10mm or more inwards of the gaurd, then you should be able to correct the problem, wether it be springs, hangers, or bent/twisted 'chassis rails' that the springs are mounted off. If the LH rear (assuming you guys are RHD) is sitting just inside the gaurd, then the problem is wheel offset.

what width wheels are you using? 6" would be the maximum you'll fit under the rear of a sedan without clearance problems IF you have the right offset. 185/60 tyres will easily fit under the rear of a sedan on the right wheels, and assuming nothing is bent out of shape.

if it is not the springs that are responsible for the diff being off-centre, replacing them with reset leaves of the same height will not fix the problem, unless they are so stiff they don't move! And raising the rear of the car up is treating the symptom, not the problem.