>I have never used Output sharpening (though I see on here that some recommend using it) b/c I want exactly what I see in Develop mode to be exactly what is exported. This has never been a problem until some time in the last 2-4 weeks.
You should only use output sharpening if you downscale your image or if you are intending the image to be printed. Downscaling introduces some blur that you can counteract with output sharpening. Printing does the same thing but to a different degree so they both need to be sharpened to correct for this.
What version of LR 4 are you using? There was one with a bug that sounds like this. Make sure you have updated to the latest LR 4. I do agree with ssprengel that it sounds like the previews are bad, but that should not matter at all for the export as that should be done from the raw directly.