Depending on where you are in the game (I figure you're pretty far considering your levels), I would suggest Nidoking or Nidoqueen as a Ground-type. Partly because of their Poison half, partly because of their moveset, but I like them.
Let's consider some other Ground options for a minute:
Call me crazy, but between those you can find one without having to hassle with trading in RSE.
Now here are my reasons for giving my suggestions.
Golem: Not really that good. Great defense, but an incredibly common weakness (4x to Water)
Onix: Same as Golem, and for Steelix: Crappy Special Defense, but you could use that...
Dugtrio: Ever since Gen 1, this is useless, basically.
Sandslash: The ability to learn other moves than Ground, but not the best by stats.
Rhydon: Another good choice, I should have mentioned it above. Same weakness as Golem though...
Marowak: Similar to Dugtrio and Sandslash, not effective anymore.
Gligar: Strange hybrid, hard to use.
Piloswine: Great hybrid, that's another possibility, but I still like Nidos better.
Quagsire: Maybe this would be okay, but its hybrid makes you very susceptible to Grass attacks.
So my final answer? Nidoqueen, Nidoking, or Rhydon.
Hope this helps!
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