Since the 4x's gearing box is designed to turn larger tires under a load, I would imagine that it is bigger. A call to your local dealership's service/parts department should get you the info you need. It isn't that big of a deal if you have to find one, they run about $150. Considering the amount of money you will be spending to do the conversion another $150 or so isn't that much. However you could reduce a lot of extra costs by finding a rolled 4x4, you can usually find them for around $500. Buying items seperately for the conversion will cost a bit more since you have to replace everything from the engine back including the rear axle(unless you want to have to have 2 spares, one 6lug, one 5lug).

As OOP's mentioned the 2wd frames are nicer for doing an SAS on, you can sqeeze a lot more tire under it w/o having to lift it a lot.
On this '92 we were able to fit 37 Krawlers with only running rear Toyota springs upfront. The axle was moved 2" forward for better clearance.