The hydraulics control the height of the body in relation to the wheels. The problem with the wheels off the ground is that the system is unloaded, so the height is probably at max. The system only pushes, gravity does the work in the opposite direction.
Use properly rated car ramps to make it easy, unless you can slide under the rear with the car on the ground. Otherwise get a skinny teenager, and be VERY careful. I did it, but it gets a bit hairy with the car lowering down on you. I don't recommend it. Height off ramps may not be height off the ground though, so you may have to fiddle.
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K