Hi and welcome to NICO Club!
I had that exact thing happen to mine. It was the counter gear set axle had snapped in half at the bearings in the center plate. I did get it rebuilt, but with parts from another tranny and with new bearings, seals and synchros from a kit. I wish I had my pictures handy and not archived. It was kind of an adventure. From running red lights and stop signs, bogging and revving, just to get it home, to taking it all apart in my apartment dining room. From inventing new tools to pull gears apart to testing my first major rebuild component. Good times! Would not do again either!
500 is expensive for a transmission. But I don't know AWD, maybe that's the norm. I don't think it fits your engine though. Different era, different bellhousing, different sensors, possibly different length. I'd look for a better match and hope for a better price.