Lately my beater '87 TII has been making a ton of diff noise on decel.
I tried changing the fluid in it to Royal Purple but that made no difference.

The amount of noise is increasing for sure.

It's completely quiet accelerating, in neutral it makes some noise, and engine braking it makes the most, especially between about 40 and 50MPH.

I've never really ripped a diff apart before, I'm thinking maybe the pinion gear is rubbing on the ring gear funny due to a worn out bearing or something? But really just grasping at straws.

Has this happened to anyone else, if so, what was wrong?

 

Mine does it...but it's supposed to. Aslong as the diff still locks and engages when it's meant to (I think stock is only 1 way, on acceleration) you're probably fine for a little while. I'd look into putting together a new one or replacing it soon though. New plates through Mazdatrix aren't too pricey.

 

Since that Tii diff is a clutch pack type did you put and LSD additive in the unit when you last changed the gear oil?

 

Sounds like the diff is just plain worn out. Check for play in the pinion and stub shafts. I bet you will find a load of backlash.

 

Sure its not Drive shaft noise?