You can buy "Cat back" exhaust systems, that come pre-bent to go over the rear axle, which is the hard part to fabricate yourself.
Cat back means from the catalytic converter to the back of the vehicle.
Your 4.0l will be a single exhaust stock, I am not familiar with the '98 system, but to switch to a true Dual exhaust would probably require another catalytic converter, you wouldn't legally have to add a 4th O2 sensor as far as I know.
Dual exhaust can squeeze a few extra "ponies" from the engine, but expense is hardly worth it with stock engine.
Extended cabs usually have longer exhaust than regular cab, so keep that in mind when looking for off the shelf parts.
Google: 1998 Ranger cat back exhaust
Local exhaust shop may surprise you with lower cost, they can bend pipe on site and do it pretty quickly, so definitely worth stopping by for an estimate.
Chain shops tend to just use/have pre-bent pipes so would be pricey, independents would be where to check.