Could be a bad throwout bearing. Be careful you can screw your whole tranny if it is. If it was sitting in there weird and then it popped back in place it could cause that feeling. And a bad throwout bearing will cause the tranny to kick out of gear do to vibrations. Also possibly a bad clutch. Do you have any slipping? Put it in third from a stop get on the gas and slowly let out the clutch. If the RPMs climb then your clutch is shot.
Could be the tranny but not likely the NV's are a pretty big powerhouse when it comes to trannys.
The main problem was the detent spring, there's a sleeve around it that holds the ball against the spring and the sleeve split dropping the ball into the pan and spewing bits of broken sleeve into everything. Fortunately no other damage was done from this, other than the spring sticking out too far which is what stuck me in second. I conveniently bent it out of the way when I forced it back out. (Hehehehe...)
Good call on the throwout bearing, since mine looks like this:

(I know... it's supposed to be completely smooth!)
The bell housing was also full of oil, including some smeared all over the clutch. Ooops. The whole thing's being rebuilt now with a synchro kit, throwout bearing, clutch, shaft seal, detent assembly and a replacement CV boot for the one that had grease coming out a small hole. Oh, and they discovered my lower rad support is rusted right out and not really supporting the crossmember that holds the whole damn tranny in. They're going to weld something in to hold it temporarily but it's back to the body shop real soon!
(Don't ask me the cost; somewhere between ouch and poing! But I've put seven times that into this car already, so in for a penny, in for a pound...)