Not a big fan of Man-it-ain't-free (Man-A-Fre) but they're in the processing building a full bolt-on 23 gal aux fuel tank.
http://www.man-a-fre.com/fjcruiser/f...iliarytank.htm
It's for the FJ but I suspect that it'll work for the 4runner easily!
Features include:
*23 gallon capacity
*100% bolt on, no drilling, welding, or cutting required
*Constructed out of high grade aluminum
*Built in skid plate for extra protection (not shown)
*Will work with most aftermarket rear bumpers
*All hardware, wiring, and plumbing will be included
*Lightweight, high clearance design
*Factory fill neck is retained for smog compliance
*Re-enforced mounting tabs
*Fully baffled
*In tank high volume fuel pump/filter with fuel trap
*Optional sending unit / gauge kit
Not a bad option but I suspect it'll be pretty $$$$. This will give you double the capacity for your expedition travels so if you've got the funds, it's definitely a great option! Give them a call and check em out!