Well the Anglo Saxons were calling the place Rodstane when the Normans turned up to write their Domesday book. And in A-S Rod sounds like it's either a measure of land or something to do with roods as in crosses you get crucified on. So I don't think rude meant rude in those days

But if it sums up your theory about the stone quite nicely (and let's face it no one knows what was in the minds of the designers, you could be right), you could say as a 21st century bit of folk etymology it's fair enough, so go for it.