24-70 sharp on the left, looks like camera shake on the right?

I've had my 24-70 for a couple of years, get good use out of it, and care for it well. I'm just noticing now, at 100% view, that the image is wonderfully clear on the left, soft on the right. But it doesn't just appear a little OOF, it actually looks the way an image does when there's just a little camera shake.

I know the short answer is it needs to be serviced, but I'm wondering if this is common, has anyone else experienced it, know what it is, and what kind of cost I may be facing?

Thanks.

picts or it didn't happen....

Could be decentering.

Tried it on a tripod?

I ask, because it just might be caused by other factors, focal plane shutter can confuse things. At the very least its worth testing to eliminate other possibilities.