Thanks guys for all the recommendations, I appreciate it.
From what you are saying, it is stuck in 4wd and not going into diff lock causing the sputtering like others have said. There are some things you can test to narrow down what your problem is.
First, pull the servo off the differential and do the two c-battery test on it to ensure it's working properly. You'll have the 3B4 servo on the 2008 so you can fast forward to that in the video if you need help testing that.
The servo should go from the F mark on the black case (2wd) to the R mark (diff lock). If it does, I'd consider it working properly for now.
Next reach inside the diff and make sure you can slide the gear back and forth with your finger, you may have to roll a front wheel a bit, but it should go freely back and forth.
If it does, I would check the relays next.
On the 07-08's you have the 5EA relay which is a black relay usually has NAIS on top with the 5EA number in small writing and you have two 5DM relays. They say Omron on top with a G8HN-1C4t number on top.. Two of the G8HN relays operate the 4wd and the 5EA should control the diff lock.
If you find your relays are functioning properly, go ahead and check your switch. There is a video on that also that you may find helpful.
See where you get after you do this, just remember, servo, relays, and switch..major 4wd electrical components. The CDI does come into play to but typically that is the last thing to mess with. Wiring also can be and is a common issue. If you get to that point, you'll want to give your wiring from the servo and relays a good check looking for breaks and tears in the wiring insulation.