I successfully rooted my shield without unlocking bootloader using towelroot. When you install the apk and run it it says it will reboot in 15 seconds, but it will take a little longer than that for some reason so just let it sit for a while. After it reboot I had the SuperSU from chainfire's website downloaded. Use root explorer to open the zip. Go to common folder and install the apk. Worked for me.towelroot.com. download.chainfire.eu/447/SuperSU/UPDATE-SuperSU-v1.99r4.zip.
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Sent from my SAMSUNG-SM-N900A using XDA Free mobile app