Had a very similar situation to this recently, causing a rough sound when opening & closing the slider.
the slider would not close without a good push (Slam) , tried a bit of Wd40 on all roller wheels which made a little improvement but still getting a rough sound when opening the slider.
what I did :
park on a slight slope, or chock the front end, (front up) spray copious amounts of WD40 into the front curving section of the mid rail channel & roller (whilst the door is held slightly ajar you can see the roller, spray upwards ).
Close the door, spray copious amount of WD40 into the entire length of the rear channel ( behind the cover panel, spraying upwards) , it’s a C Section runner, which can’t been seen properly whilst the cover is fitted, so you need to spray upwards to flush the rail Fully, slide the door back 3/4 distance spray again directly into the channel & at the roller, from the front of the door.
the resultant excessive fluid / crud will flood out of the cover panel nearest the tailgate, it will be grey and gritty, like metal filings ( use kitchen towel to prevent the excess cascading down the paintwork).
activate the door back and forth, several times, you should hear an improvement and feel less resistance, repeat the WD40 flush as necessary,
then jetwash the rail from front to back by spraying directly into the Channel gap / roller mech. (Door fully closed)
open the door , close the door and repeat the wash until you are happy with the improvement.
then squirt a very small amount of WD40 onto the mid rail roller wheel(s) and operate the door several times (paper towel at hand)
result = door opens & closes like it’s new, no force required to close the slider, it simply slides into the power close with no effort.