Here are some links that explain the issue:
Lightroom can do a very good job processing High ISO images, but you need to use the proper control settings.
Only make Sharpening and Noise Reduction settings at 1:1 Zoom View.
1) Start by adjusting the Sharpening until it looks good, but NOT over-sharpened. Sharpening amount should be no more than 50 to 80 at this point. Next adjust the Noise Reduction controls.
2) Move the Color slider slowly and stop at the point you see all color noise removed. Try increasing the Smoothness slider to see if it improves the color noise removal.
3) Next Increase the Luminance slider until the remaining noise is reduced so as not to appear obtrusive.
4) Go back to the Sharpening Amount slider and increase it slightly to restore image sharpness. Use the Masking slider to reduce noise in even-toned areas. You may need to increase the Sharpening Amount after using the Masking slider, but don't over-do it!
5) Go back to the Noise Reduction controls and adjust Luminance slider and its Detail slider to fine-tune the NR correction.
The Noise Reduction controls and Sharpening controls interact - It's a fine-balance of obtaining low-noise with "reasonable" sharpening. With High ISO or simply "noisy" image files there is always a trade-off of sharpening loss for low noise. Keep in mind image files are never displayed and viewed at 1:1 Zoom in a real-life scenario. Export or Print Output Sharpening should be used achieve the final image sharpness you desire....not the Develop module's Sharpening controls. Trust me it works!
One last tip is to achieve more accurate image previews in the Library module is to use the 1:16, 1:8, 1:4, 1:3, 1;2 pyramid preview ZOOM settings. When using Fit or Fill a 2nd interpolation is required, which can slightly soften the image. The only 100% accurate view in LR is the Develop module 1:1 Zoom view, which is why that should be used for making all Sharpening and Noise Reduction adjustments.
Thanks for the quick response. Im not having an issue with the Library and Develop not matching, because they do. My problem is if I turn the Luminance up and turn the detail up, the cool results I see on the screen will not translate to an equally cool looking jpg after exporting. I never had this issue with 5.7. The only way I can export a change is to turn the luminance up and the detail down, which leaves me with a gooey looking image.Maybe I'm doing something wrong, but I know this feature does not work in the same way that it did in version 5.7.
Are you using 'Resize to Fit' on export? If so are you applying Output Sharpening? If Output Sharpening doesn't fix your issue please post a screenshot of your Detail panel settings as below and let us know what ISO setting was used for the image file in question.
You can also try disabling 'Use Graphics Processor' in LR Preferences if the above suggestions do not fix the issue.
Don't discount the advice deepakgahlot posted. Even if Library and Develop aren't mismatched, the advice is still entirely valid. In other words if you're not judging your NR at 100% zoom, what is exported won't look the nearly same and you should be judging it at 100% zoom as well.
Forgot - the ISO is 6400. Thanks again for your help
Thank you for posting the requested screenshots and ISO setting. You are seeing more noise in the exported JPEG image file than in the Library and Develop modules because you have Output Sharpening applied (Sharpen for: Screen High). This applies additional sharpening not present in the Library and Develop modules. ERGO: They will look different. Output Sharpening should only be used when reducing the image size with 'Resize to Fit' checked. Any time an image is resized it loses edge sharpness and Output Sharpening restores edge detail in the reduced size file.
Follow the instructions deepakgahlot provided in reply #1 and in the Export module make sure Output Sharpening> Sharpen for: is not checked. If you are still having issues please upload one of the image files to Dropbox or other file sharing site and post the share link.
That did it. Thanks for your help and patience.