I am a new member and have an XJ-SC. I have. Just replaced the front quarter light window seals and the correct way is to remove the frame that supports the window. The screws that hold the frame are shown in an original repair manual. Sealant will make it slightly difficult to pull the frame away but I think is easily done. The longest job is cleaning off the old sealant.
One major thing to think about is the condition of the quarter light support plates that fit at the bottom of the glass that are attached to the door. These are usually very rusted and need replacing. I managed to get 2 from a breakers yard as they are difficult to find. I have 1 reasonable spare one if needed.
If this any help please contact me for more info.
Hope you can read this procedure.