The rear door hinges really need grease nipples fitting, as once they get rusty inside, you cannot get any lubricant to the pins.
We've been dosing ours up weekly with WD-40 and it is slowly working its way inside.
The door stay as BoldreChris mentioned needs attention, and the stay pin at the back of the car needs to be kept oiled/greased, as does the sliding part under the door.
When I get a minute I'm going to drill and tap our two hinges for grease nipples to see if we can force some grease through.
Peter