Play source upstream from driveshaft?!
 

2007 G35 coupe (non-HR, NA), 35k miles, 6-speed, quaife LSD, 4.08FD.
Developed some play in drivetrain after practice day at autox course (like 50 miles)- felt very similar to worn u-joint or excessive ring/pinion play. Was so sure it was coming from recently installed FD gears that I pulled pumpkin, but I was mistaken- play there is within spec. Not coming from driveshaft joints either- play is evident at tranny output. Trying to picture how a pressure plate might wear/fail in a way that could cause a small amount of rotational play, but doesn't seem likely. So, I'm guessing I trashed something (or at least threw something out of whack) in the tranny. Anyone hazard a guess what I'm dealing with?
TIA,
Ethan