We have been struggling with the same problem. It so happens that the boneheaded home repair guy (me) walked into the screen which knocked it off the bottom track. After that, the door was hanging by one roller, and the other plastic piece was hanging adrift. Aha! Now we know! Here's the trick - I could not lift the screen off the bottom track, and the ends of the top are closed off. There are four screws that hold the top rail in place, with the ends also holding a plastic bumper. Take out all four screws, and you can carefully remove the top track, slide it off, and replace your stripped #
84-105. PAY ATTENTION TO ORIENTATION OF PARTS!
To reinstall, replace the bad #
84-105, slide the top guide over the rollers, place the door on the bottom track, then loosely replace the four top screws. After all four screws are in, tighten them. Check the door for smooth function, brag about what a superstar you are, and get some refreshments!
Pella's website was useless, but the part showed on an image search. Parts ordered from SWISCO!