I'm not particularly knowledgable about the mechanicals in these 4WDs, but I don't think that's the correct understanding about the purpose of the fuse for the actuator.
If the actuator fails in such a way that it is drawing more than normal current, then the wiring can become heated and cause more drastic problems
due to the extra current (which causes heat). The fuse is there to insure that the wires never see more current than they are designed to safely handle, and if the actuator fails this way, it CAN cause the fuse to blow too.
On the other hand, if the actuator just fails from some internal problem and simply stops drawing any current because of the failure, then the fuse will likely not even know it has failed. An example might be if the actuator gets hot from heavy loads or external heat (exhaust, maybe?) and the internal wiring in the motor winding just opens up suddenly. The actuator doesn't work anymore, but there is no extra load being drawn to blow the fuse.
In the case of your actuator, this may be the case. You mention getting pulled out of the mud, so my first thought is that the actuator gets muddy from your use of the truck and you shift in and out of 4WD during these times to only engage it when you really need it. If this is the case, the layer of mud may be holding the extra heat inside the unit from the extra usage, and you end up having the actuator just "burn up" inside itself.
Obviously, these are all just guesses, but based on what you've said and what I can understand about the system, you should look closer at these factors and decide if you need to somehow add protection for the actuator - maybe a shield to keep mud off of it.
On the other hand, the first replacement might have just been faulty. But at some point in the multiple replacements, we all have to decide if we have a cause at work making it fail over and over again and look for the cause.
Good luck!