http://www.the12volt.com/caraudio/woofer_configurationsm.asp?Q=2&I=22#_index
If I'm reading that correctly, you can only wire 2 dual 2 ohm subs at .5, 2 and 8 ohm
To give you an idea of a budget build see my sig.
hu --- 140.00 new
Install kit and wiring 40.00
Subs - 215.00 used
Amp- 123.00 new
Amp wiring 26.00 new
built my own box so 40.00 in materials
And still looking for front and rear comps.
Dont worry about it, like I said I work at a dealership and my second appointment was a grand Cherokee and I just measured it up, only took 30 seconds.
I measured a 2011 this morning.
41.5 in between the wheel wells
about 18.5 to the window line
29 to the back of the rear seats without pressing too hard
56 to the back of the front seats with seat back pretty far
This might give you some clues before some one can measure
Interior Cargo Dimensions - JeepGarage.Org - We Are Jeep!
Hope link works. Hate linking on iPod touch