From the summit of North Palisade, the traverse to the top of the U Notch is class 3. Then you can downclimb to the U notch, class 4, or do one rappel.

I would just stay on the safe side and rap the last little bit to U-Notch. I don't remember if I used a rap station or set up my own, but always be prepared to set up your own.

First my disclosure, I have gone up the chimney from the U-notch to North Pal, then we went on from North Pal to Starlight. So I have been on the route, ascending, but have not rapped down or downclimbed it.

The chimney is two pitches and there is a good belay about half way. Its pretty steep but its good rock with plenty of good holds, really fun climbing. My friend led the chimney and took off his pack for the two pitches. We hauled his pack up on the rope, if that gives some indication of how vertical it is. Or take a look at this angle looking up from the notch. (there are more good pictures on the SP page for the U-notch route to North Pal).



Here’s the description from the SP page on the U-notch route:
“On the right (west) side of the notch there is a steep wall with a crack/chimney system. Opinions differ on the difficulty of this section. Estimates range from Class 4 (old school) to 5.6 (grade inflation?). One thing is for sure, it will feel harder in mountaineering boots, and even harder if wet. Plan accordingly.”

Here’s the option to avoid the chimney, from the same page:
“There is an easier alternative to the crack/chimney system above the notch. Descend the couloir on the south side from the notch about 100' until it is possible to traverse climber's left (west) around a rib on ledges. This will lead to a gully and Class 3/4 scrambling up to the ridge, where it rejoins the normal route.”

The chimney that RickF is mentioning can be rappelled. It's two rappels. I've used a 60m rope for the last few years in the Palisades although I think the rappels would even work with a 50m. The first rappel has a lot of webbing on it. The second rappel is easily found. A nice ledge divides the two pitches. I'm always ready to add a new one and subtract old slings if need be like Marcsoltan says above.