4 speed ka-booom !

Yup, can check without needle bearings. Just set box on it's head. I've never seen selective washers. I've never seen one not dial in with new washers.Or with good used ones for that matter.
Endplay is .015 to .029

From post 15; But 30 passes on a small 8.75, and a noisy one at that; I would be looking into her. Especially since you say it worked in reverse, which drives the pinion backwards. I suspect the pinion nut worked loose and it climbed into the ring-gear.

When you drive the pinion backwards, as in reversing, the pinion if loose will be forced forwards out of the ring gear, making movement possible. When driving it in a forward gear, the pinion will walk into the ring gear, jamming it up.