Annnnnnnndddd it still doesn't work!!!! HAH!!! Took it out and it it didn't fix it. I got bored and pulled off the 3.5 inch pulley, loosened the spring, and installed the spare 4.5 inch pulley to see if that would make things more stable. I ordered a new kevlar belt. Reassembled the mule drive and fired it up.
Now the belt stays right side up on the PTO pulley, the tensioner pulley, and the mower deck pulley, but it FLIPS on the Idler pulley. Upon closer inspection of that idler pulley- the inside of the V groove has worn significantly from a "V" shape into a rounded "U" shape over time. I could see where it wouldn't guide the V belt into a V configuration. The belt is also probably trashed at this point.
So after a quick internet search of a 4.5 diameter pulley and a 5/8 bore sealed bearing- I found TWO new pulleys at tractor supply down the street. I'm off to pick them up now. Stay tuned! :)
Edited by dualresponse1731, May 21, 2016 - 02:11 PM.