I've been farming 75-76 rifts in group and 72 solo with a Seismic Slam Earth build, but I've stopped playing for quite a while now so not really had the chance to push with it. In a group the best you can do is debuff mobs/buff the group while being a medium source of both AOE/single target damage and very very high toughness.
It's not very high on damage though so you're never going to be pushing strong damage with it compared to other specs, Raekors while it can make the most of Vile Ward shoulders during AOE does way more damage. On Rift bosses I personally have seen over 150billion crits regularly with each Seismic Slam (and way over 200bil maximum), but that's during Convention of Elements, take into account that some other classes are doing multiple times more than that.
Seismic Slam is really similar to playing Blade of the Tribes and running Boulder Toss, I think SS is way more fun to play but it suffers the same lack of damage as you push into the high 70 or approaching 80 rifts.
This is the build, I'd guess with ancient gear, extra stats added (consuming legendary gems) and a decent Paragon you could push past GR80 solo, but with the same effort you could push a lot higher with Raekor.