Congrats on your new ride!
There's a pretty big thread on the Nitto TG's here:
20" Nitto Terra Grapplers
I'm running them OEM optional size 265/50r20 and like them, though they are more noisy as expected than the stock 18s I took off, but not bad. I posted a few pics in that thread.
The ride is a bit harder than the 18s too but if you went to 275/55 as you mentioned it would probably be better than 265/50, and more off road worthy