The locker does not affect the backlash,
my best guess is the setting that the
gears were at has changed, as in the lock
on the ring came off and is turning

or the setup was loose and now it's wearing
in some and has increased backlash.
I don't think in a month of driving gear wear
is a factor unless your gears are eating
themselvs up, which is a bad thing

I know the crush sleve has to be torqued
properly, and that keeps the preload on
the pinion, if this was not done right it might
be moving the pinion around, bad thing as
that eats gears fast
You also said you now have a vibration in
the driveline, which sounds like the pinion
could be loose from improper install, who did
the installation of your gears

You ??
if not get back there and get it fixxed before
you eat a set of $300 gears, and if you did it
then you should recheck the torque and backlash
right away, yes I know it's a PITA but it's got to
be somthing wrong and it can't stay like that.
Good luck,
Darrin