Your transmission system conveys power from the engine, making it one of the most important parts of your vehicle. Such systems are very complex and placed under considerable strain, so they are vulnerable to wear and tear. This is even more likely when dealing with a 4-wheel drive (4WD) transmission because those systems are also tasked with distributing power between the front and front wheels through an integrated transfer care. Additionally, 4WD vehicles tend to be put through more heavy-duty use.
If your transmission does fail, you may be given the option to have it rebuilt. This means it is taken apart and cleaned, with new components replacing problem ones before the system is put back together. It's often a good option, but here are just a few reasons why you might want to consider having your 4 wheel drive transmission replaced instead of rebuilt.
4WD vehicles generally demand greater reliability. They are more likely to be used for commercial purposes, in which case a breakdown can put the business in trouble, or for venturing across the backcountry, in which case a breakdown will be far more inconvenient, and possibly dangerous – you can't exactly call for a tow truck when you're in the middle of nowhere.
A new transmission is simply more reliable than a rebuilt one. The issue with a rebuilt transmission is that only the main problem components will have been fixed. That's fine if only one or two components were broken, but it's more likely that general wear and tear simply resulting in a few components breaking down earlier than the rest. A 4WD transmission needs to cope with everything from crawling slowly up rocky inclines in first gear to towing heavy trailers at highway speed, so transmission components are more apt to wear down. If yours fails once, going for a replacement instead of a rebuild makes it less likely you'll suffer a second breakdown.
If you need further proof that a new transmission is more reliable, just compare their warranties to those of rebuilt systems. In nearly all cases, you'll find that new transmissions come with full warranties, while a rebuilt one will enjoy less substantial coverage. That's not just important as a sign of reliability. If your transmission fails, you want to know that costs will be covered, especially with the more expensive systems used in a 4WD vehicle.
As mentioned above, 4WD vehicles are more likely to be used for commercial purposes, whether that means towing horse trailers or carrying construction supplies. If this is the case, you want to get going again as quickly as possible. A transmission rebuild takes time. You want to get back on the road in as little time as possible, a replacement is where it's at.