3.21 or 3.92 are the only options on the 8 speed.
For 2014+ with the Hemi, you are correct.
With the 3.6 V6, you can get any of the three gear ratios.
With the 3.0 EcoDiesel, you can get either 3.55 gears or 3.92, 3.21s aren't offered.
For 2013, first year the 8-speed was offered, you could get any of the three gear ratios with the Hemi.
The 8-speeds with the Hemi or EcoDiesel have been pretty reliable since day 1, as much as any modern transmission can be. The only common issues so far have been programming ones, if there are any at all.
The Jeeps with the Hemi and the 8-speed, first done in the 2014 model year, have the same record. Very few issues, and the few that do, its usually programming related. The 3.6L V6 and its 8-speed are a different story, reliability seems to be all over the place, although many argue that is simply because the V6 is built/ordered in higher numbers with the Jeeps (something like 80+% of all Grand Cherokees sold today are V6 powered).
The 8-speed for the Hemi and EcoDiesel are made by the Germans, even if they were assembled here in the USA, and have been used for a while now on many high end European vehicles. The 8-speed in the 3.6 is a modified German design, built and assembled by FCA.
https://en.wikipedia.org/wiki/ZF_8HP_transmission
From what I've seen, many of the reliability "hits" against the 8-speed are simply people used to the older 5 or 6 speed transmissions. They expect that sort of feel from the 8-speed, and when its different, even though working as designed, they keep taking it back to the dealership stating its not acting right.
