Maybe this will help: Jeep Grand Cherokee WK - Spark plugs
According to the 2008 Owners Manual, page 446, the plugs on top (intake side) are standard grade and need replacing after 30K miles. The plugs on the bottom/side (exhaust side) are premium grade and only need changing after 102K miles.
There are 16 plugs total: 8 on top (intake) and 8 on bottom/side (exhaust).
It looks like the top plugs are Bosch Nickel Yttrium (0.040 gap) and the bottom plugs are Bosch Iridium (0.050 gap). I'd stick with those plugs since they're what your Jeep came with from the factory. Torque specs for all plugs is 20 ft/lbs (be careful not to overtighten).
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