TuphRyder, I am running 37x12.5's on only a 4" Rancho lift. Can it be done, by all means. The question is, are you ready for a few modifications?
The first one is to simply cut the jack nub off of the lower A-arm. I found that I was getting the tire to rub this when at full lock in the steering.
The second modification is to remove the front wheel well liners and then using a 3 pound hammer, you need to bend over the seam that joints the firewall to the rest of the body. You only need to bend over the bottom 9" or so of the seam. Then you will need to use a bolt to hold the fender liner back and out of the way by drilling a hole through the fender liner and then into the folded over tab so you can run a bolt with some fender washers through all of this. Mounting a rubber washer between the fender liner and the body is a good idea as this will help minimize any noise you get from things striking the fender liner now.
The final thing that you will more than likely need to do is to modify the back edges of the front fender. I used a pneumatic angle grinder and cut off about 1" of the bumper on the bottom corner and then tapered it to essentially nothing at the top of the bumper. This is needed to give clearance for the tires.
Please keep in mind that this assumes that you are running an 8" wide rim with 4.5" of backspacing. Running a wider rim is only to garantee even more modifications will need to be performed. Things are a very tight fit with what you are planning. With my 4" lift, I tuck the rear tires and it is to the point that I can not slide my finger between the tire and the body all the way around the wheel well. The front tires only miss the back of the fender liner by like 1/4" in the worst case position (roughly half way between straight ahead and full lock). You may have a little more room in the rear (not much) because the Fabtech kit really is a lift/levelling kit. So, the kit really is a 5.5" lift in the front and a 4" lift in the rear.
Here is a pic of what you can expect your truck to look like:
When the truck is twisted all the way up, this is what your rear end is more than likely going to look like if you use the rims I say:
If you have any more questions, just ask. I have been down this road already (been running the setup for 6 years now). Please keep in mind that this is assuming worst case conditions. If you are only after a street queen, then the limits can be backed off of some, but not much.