couple of Pickup/Tacoma questions

I'm thinking of getting a cheap Pickup/Tacoma (about 89-96) and have a couple of questions:

Can a 4wd truck get the mileage of a 2wd when you don't engage the 4wd?

On the extended cabs, what is underneath the back seat? Is it something that could be removed or altered to accomodate a big locking storage box?

I have 2004 taco 4x4 v6 and it gets good gas mileage 20-21 mpg, sticker says 19 but I dont drive it hard. as far as the ext cab goes, the seats on mine fold up and out of the way, you could put a storage box in there easily

I have a 98 taco 4x4 v6 with the jump seats, still drives like it did the day i bought it, seats fold up, under the seat behind the driver is a small storage unit, under the seat behind the passenager is a storage box just slightly larger than that behind the driver, both could be modified to lock, good luck

what's disadvatages does the Tacoma have if any? also, what year seems to be more durable?

There was a head gasket issue w/ the 96's..my vote is 01 n up....but I'm biased cuz I got an 02