The level is set with the level control valve at the differential. You will see a turnbuckle looking device connecting the control valve to the sway bar. I am not sure if moving the valve actuator up raises the car, or vice versa. Just adjust the turnbuckle a couple of turns in one direction. Car goes up, turn the other way. Car goes down, you are done.
As far as losing fluid, I would not guess the nitrogen accumulators, it seems it is usually at a fitting or line vibrating loose. I had a leak from the high pressure line, just behind the smog pump, where the line rubbed the engine block and wore through. It leaves the passenger side of the block wet, inside the engine mount. The fluid is clear and slick, and would leave a trail someplace of it is leaking.
If the rear end raises when the reservoir is full of fluid, I think the nitrogen accumulators would have to be good. At least one of them. They are not a dealer only part, I am sure Partsshop will sell them to you, much cheaper than a dealer will. I posted my experience replacing them. They are difficult to get to from the floor, much easier with a lift.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)