Hey Kief, I do hope all is well my friend. I have been dealing with the famous "ClUNK" sound while downshifting from 2nd to 1st also. I had my Guibo replaced about a week ago and still am experiencing this same harsh downshift. Since the "Guibo replacement it does feel better but it only decreased what used to be a very loud "CLUNK & HARSH DOWNSHIFT" to a downshift that's now not as "LOUD AND HARSH BUT STILL VERY NOTICEABLE".
After looking at you DYI on the Guibo i noticed that there are a few other parts besides the guibo that may cause slack in the drive train. Could the center bearing, driveshaft, transmission mount possibly cause the slackage within the drivetrain and whats the possibility that my driveshaft is bad???
Replaced so far: rear trunk latch and actuator along with cable, front driver side window regulator, reprogrammed key fob, front trunk hood cables, front head light and fog lights, valve cover seals and gaskets plus timing cover gaskets