I am almost done putting the transmission tailshaft back on my 1990 Volvo 740GLE after a rear tail seal leak. When I was removing the flange, I remember the directions said it would be necessary to put the car into neutral in order to rotate the driveshaft so that all the bolts could be accessed. I did so. I also put it back into park for the rest of the repair.

Well, I am at the step where I need to tighten said bolts on the flange, so I put the car into neutral. The driveshaft won't rotate, but the rear tires will (??). Actually the rear tires will rotate in D, R, or even Park, but the driveshaft won't move. What am I missing here? The shift linkage is reattached. Did I knock something loose in the process?

I don't want to go forward and reattach the tranny mount and crossmember until I know I can move forward without having to go back and take it all apart again...