The latest stable version of CuraLE is 3.6.20 and includes Marlin 1.1.9.34 firmware for the TAZ 6. This is the latest “supported” version from LulzBot. CuraLE 3.6.20 also includes 2.0.0.144 for the TAZ 6 but I don’t believe it is considered stable. CuraLE 3.6.20 does include Marlin 2.0.0.144 (only) for the Workhorse and Pro.
You can also find (up to) 2.0.0.174 on http://devel.lulzbot.com/software/Marlin/. I have not tried any of those versions on my TAZ 6.
See https://forum.lulzbot.com/t/taz-6-latest-firmware-is/21413/21 for another discussion. Towards the end of that thread is a link to the Drunken Octopus repository created by the former LulzBot firmware developer. For the time being, this would be my choice for how to get Marlin 2 working on my TAZ 6.