Welding the rear accomplishes the same thing as a spool without the cost (or a locker without being able to unlock). Both wheels will always be spinning together. That's great for rock crawling/mud, but not so good in a lot of other (higher traction) situations since the back of the truck always wants to keep going straight even when the front end is turning.
This is not something you'd want to do if you need to use the truck on the street much and can't spend a ton of money on replacing worn out rear tires. If your truck is only used for off roading and you're on a budget, go for it. For a multi-purpose truck however, you generally want a selectable locker or limited slip so the truck still works normally on the street, but has better traction off road.
