As an owner of a YJ for 5 years, and now a CJ and JK, I can say that besides the better frame and springs, the YJ has absolutely no benefit to it mechanicly over a CJ or any other Jeep.
The rear axle is the same crappy Dana 35 axle used in the rear of TJs, the AMC 20 in the CJ is a far better rear end, the Aisin transmissions aren't as good as the net would have you to believe (I've had both the AX5 and AX15, both sucked).
I've had 6 Jeeps in my time, and I had my fun with the YJ, but it was the only one to give me non-stop problem after problem, and it was the least capable Jeep I've owned.
Unless you gut the thing and replace just about everything, its not worth it. In fact, it'd be better to just swap the front clip and fenders onto the CJ if you really want that rare front end. Otherwise, sell it on and get to work on the CJ.
As for the grill swaps, this has been done loads by people over on Jeepforum, but they usually swap the entire front clip (grill, hood, and fenders) because the YJ and CJ tubs are nearly identical.