I HAD the tank where the spare used to be...I moved it, as its not as tucked up as I thought it was going to be...I have a 4 gal tank though...the 2.5's may tuck better.
If the 2.5 gal tanks are less than 6" diam....they MIGHT fit into the hollow part of the rear bumper...which would also protect them.

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- TJ
2001 Xterra '03 VG33, SE 5 spd, 305/70/16's, Revolvers, UBSkidderz, Doubled AAL's, 3"SL/2"BL, winch/bumpers, skids, sliders, OBA, Snorkel, pine stripes....
Friends don't let friends drive stock.
http://www.gifsoup.com/view/501230/tj-tackling-crawlers-ridge-o.gif