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I was having starter problems on my ranger when I saw this. Looks like I cant post pictures. The frame is broken up against the right front shock/spring/motor cross mount.
Rusted as in surface rust and the steel just happened to crack for whatever reason?
Or rusted as in the frame's rotted out and you can stab a hole in it using just a screwdriver?
If the metal is still solid and simply cracked from stress over the years, just weld it. That's what I did with my cracked engine cradle/crossmember. If the crack stops somewhere, drill a hole exactly at the end of it (both ends if required) to stop it from spreading before you do anything else, and right before you weld it, vee the crack out along it's entire length to help with weld penetration.
If your frame is rusted to the point where it's just rotten and falling apart, then there ain't anything you can do to save it. You'll have to find a donor truck with a good frame and swap parts around to make one good truck out of 2. Look for a wrecked one with no frame damage, or one with a blown engine or something, they can be had cheap.