The end of an era. Or rather, the end of many errors.Sounded completely seamless, and the interpretation of the joke wasn't impeded in the least by the r-ful vs. r-less difference. (The map, from here, shows areas where white Americans have been reported to be r-less.) That is, for the joke to work, you have to be able to interpret the last word of the first sentence as error, as said by a speaker who doesn't produce full r sounds at the end of syllables, even though you do get an r before the s at the end of the second sentence. But then, it seems like everybody gets the joke with the name r-ful dialects, where the humor is in the possible confusion with awful dialects.
OK, it's true: Nothing ruins a joke like explaining it.