on my 99 durango 5.9L 4x4 it has a cracks on the frame were the streering box is mounted. how can i fix that, is there an kit or something.

 

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Welcome to the car world! I am glad you decided to learn a bit more about your vehicle. You have three options (they are ranked with the first one being a tiny crack and least expensive to make crack and a good chunk of change):
Weld the crack
Patch the crack (basically weld another frame around the crack)
Or replace that part of the frame.

 

If you choose to weld it, you could add a pinion nose brace. They are used a lot on Jeeps when larger tires forced against rocks make steering forces higher. Here is a photo on the one I made for my Jeep. It's a simple exhaust clamp with a rod welded on. The other end of the rod is bolted to a hard spot on the frame on the other side.