Remember DX mode is a "crop" mode. So all you need to do is shoot in FX and crop in post to the image you want. You do not pull the image any closer, you just change the feild of view to match a longer lens. Your 17-35 is still a 17-35 and the same for the 28-200. You will get a smaller file in DX mode on an FX camera but you give up the flexibility as to where your crop is.