get yourself some lube, either WD40 or spray grease or something and spray loads of it in there, try and get it under the rubber. then get a big flat blade screwdriver (BIG) and put it under the rubber and work it loose. Just takes a bit of elbow grease I have done this without jacking the car up at all before so its definitely possible if you have it on some sturdy stands
EDIT, and best put something under the back box to support it, an upturned bucket or whatever as its a bit heavy with the awkward angle you will be holding it under the car when you get it free.