Half solutions are bad solutions
Changing the rear shock alone is a very bad choice. It changes the geometry of the bike. Every mechanic will agree. As Glencoeman said, you need to raise the front fork as well, but even this solution is not wise. To lower the bike, you should change both front and rear suspension with approved and tested products. The best solution is offered by Wilbers (Germany) allowing you to chose between 20, 40 and 60mm lowering: complete rear shock and matching front fork springs. The second best is Hyperpro. RalyRaidís solution: shorter shock, raising front fork, is as expensive as a Wilbers solution and far less recommended. They offer this solution to people, but their main specialty is their complete suspension for real off-road use, which is, indeed, one of a kind. Remember to shorten the side stand!