Rolls forward in neutral, locks up rolling backwards??

Howdy all, I've run into a situation here with my 71 Scamp. I have a 904, and at the end stages of an engine rebuild. The engine is in and connected to the trans, shifter linkage is disconnected. I have a 7-1/4 open rear.

My issue is that I'm not able to roll it backwards when it is in neutral. It rolls forward with maybe a little resistance (hard to tell since I'm on a slight incline and I'm just one man pushing a car), but will lock up when I try to roll it backwards. It feels like it is in park when trying to roll backwards. Both wheels move freely when in the air.

There's no fluid in the transmission currently, and when I had the pan off to change the filter/gasket, I noticed two things. Brass colored glitter, and black chunks of who-knows-what. I attributed the brass glitter to the front bushing that had been worn, since I saw the same glitter at the bottom of the bellhousing when I changed the front seal. But the black chunks? Either way, these might not even be related to the problem at hand.

It's been suggested that maybe I have a bad sprag or a problem in the differential. Does anyone have any thoughts?