So I'm guessing that I'm not the only guy to do a write up on Frame Stiffeners but I took the time to take some pretty good pictures so here goes nothing!
Due to many articles/posts that I have read over the past month of owning my XJ, I decided to install some frame stiffeners to 1.) eliminate a lot of road noise (creaks, rattles, groans, the normal older jeep noised), 2.) to prevent the possibility of twisting my frame, and 3.) elongate the life of my XJ uni-body (I hope)! I decided, after much debate, to go with the XJ frame stiffeners from TNT customs. The frame stiffeners came in 2 days after I ordered with no hiccups and I began to work. 
My first order of business was to grind the paint and gunk off of my stock frame. It took me about an hour per side using a hard grinder wheel followed up by a soft grinding pad to smooth everything out. As in all things, when you think that you have prepped enough, you haven't! I did the grinding in 2 sessions because it is a bit of dirty, hot, work.
Before: 
After:
My first concern with the stiffeners was the possibility of moisture, leading to rust, getting between the stock frame and the frame stiffeners. So after doing a bit of homework I ordered a product called "U-POL Weld #2". 
After reading reviews this appeared to be the only weldable primer that was worth the money (a whopping $17 a can from amazon). I coated the inside part of my stiffeners (the part that would be facing the frame) and the entire stock frame that had been ground to bare metal. 
A couple days later (today) I took my jeep and stiffeners to the welding shop where I work and started the fitting and welding process. The TNT Frame Stiffeners fit great with only one issue, there was no holes drilled for the brake cable bracket mounted on the driver's side stock frame. So after a quick test fit and marking for the location of the holes, I put the stiffener on the drill press, drilled 2 holes, and mounted them back for installation. (Sorry, no pics of this before painting)
A word of advice to anybody who decided to go with the weldable primer route, I would highly recommend that you do not overcoat the upper part of the stock frame. I had a ton of trouble getting a good bead started. Even though this weldable primer that I used got great reviews, I still had to fight it to make good welds. All in all, the welding took a little over 3 hours (working by myself), but the end results turned out great! After letting the frame cool for about an hour I applied 2 coats of POR-15 rust preventative paint. 
I used this stuff at the recommendation of a buddy of mine who builds BMW race cars for Dupont (he knows a thing or two). He says this is what they coat all of their cars under bodies with and it works great. It went on very easy with a paint brush. I purchased a quart from amazon ($30) when I got the primer. 
All in all, I was very pleased with how this project and I can already tell a huge difference in the amount of road noise that I hear when driving on the highway. If anybody has any questions feel free to message me or post on this thread. Thanks! -Owen Frederick