I don't know a lot about the Nissan automatics, but I've seen a lot of automatic Pathys on craigslist with transmission problems. My experience working on an automatic trans is pretty much limited to Ford AOD-E/4R70Ws, but at a guess I'd say it's a shift solenoid (assuming it's electronically controlled). I could be wrong, though. Have you considered having the local Nissan dealership look at it?
I've seen a lot of shady shops in several parts of the country, which is why I do as much work on my own vehicles as I can.