I've just started driving a 1998 Safari. The sliding side door was really hard to open and close. After suggestions here to use a silicone spray instead of WD 40 I stiil wasn't sure because there were so many at WalMart. I asked at my mechanic's shop and was told to use P B Blaster, it's about $3 at Wally World. First I cleaned the track by sweeping it really good. I sparyed it only once and slid the door opened and closed a few times and it was GREAT. I can open and close the door with one hand. I have carpal tunnel, arthritis, and other nerve issues with my hands, also my collarbone on one side is dislocated and fused that way so I really needed help. The back door latch seems to be the issue there. This worked well for me.
Good Luck
Jewellann