Check the actual shocks in the back before you decide that the car has the LAD shocks - if there are no high-pressure hydraulic fittings, you'll need springs to do the lowering. If it is the LAD rear, you can adjust the levelling arm that George described, which will drop the rear a bit. Simply removing fluid will not accomplish this.
Another note - when Drew Zacharda did his (1994) 530iT => 540iT/6 buildout, he replaced the shocks and was looking to lower the rear. When he pulled the (non-LAD) rears out of his wagon, he found that the springs installed in his car were already the part numbers listed for the factory's "Low Slung Sport Suspension" springs. (
See Link here for part numbers) So you may already have a moderately 'lowered' rear end, but you won't know unless you can look at those springs up close.
Which brings me to the last note - Most people feel that the FRONT of a Touring is what needs to be dropped - even with 15" wheels, it often looks like it's dragging it's rump on the pavement. With the pre-installed "Low Slungs" in Drew's car, and the factory "Low Slungs" in the front, plus the 3mm spring pads that give you another 1/4 inch drop up front, the 540iT/6 looks quite well balanced...... Dropping the rear without dropping the front will definitely look a bit unbalanced.