Worn out diff??? How to tell??

[meet07]

I have searched yet but im wondering how to tell if a diff is starting to get a lil lazy. When i try to preload the transmission I can hear a whinning noise. I know its in the drivetrain somewhere.

car is a 89 240sx with a rb25, upgraded clutch and flywheel with a j30 diff.

im thinking about swaping back over to stock and welding the diff..

oh yeah she is still laying down dual marks...