Despite a warning on the side of the transmission shipping box, FedEx had it standing on its end when I picked it up. A note inside the box indicated that this can cause the transmission to lock up. Sure enough, I couldn’t rotate the input shaft; it would just rock back and forth but was clearly binding. Fortunately, there are instructions on their side about how to fix this.
I started by removing the center cover. This was stuck down with gasket maker and it was a pain to get off and clean up the excess sealant. Once the cover was off, it was obvious what the problem was. Despite the gearshift lever being in neutral, one of the shifter linkages was engaged. You can see the top linkage is closer to the forward (left) face of the cavity.
Popping this back freed up the transmission and the input shaft could now rotate freely.
I reapplied some gasket maker and reattached the cover. There is a good chance I’m going to take this off again anyway since we’re mostly likely going to move to the mid-shift position.