I installed a eaton true trac unit on my firebird and have a excessive clunking sound every time I shift . it feels like there is a lot of slop at the internals of the diff
but when i turn the driveshaft with the wheels on the floor it is only like 8mm left to right
backlash is set correctly and the rear is silent on the highway
u joints are also new so as trans mount and suspension bushings
i greased the splines of the driveshaft witch goes into the trans with a heavy duty grease, it helped a little bit
used 85w90 gl5 mineral oil , should i try a thicker sae140 ? i only drive in the summer
hope someone can help thanks

 

Thicker lube won't benefit or protect anything better in a 3500lb light-duty application.

Easiest to troubleshoot, with most likelihood to cuase clunking? Trans. mount. Poly especially can transmit more NVH than rubber. Trailing arm bushings can too, but they're farther away, and more work to change.