Frame cracked

I would fish plate the cracked area of frame so truck could continue working but looks like the unit just not up to the task.

 

Slow down the truck on off road?

 

That's a pretty wicked crack. May not be a bad idea to consider a re-rail with new 3/8" rails.

 

From the rust at the cracks that did not just happen. A leaking air bag could likely have allowed the axle to excessively beat on the stop causing more damage.
I used to see a lot of that kind of damage on fertilizer spreader trucks. We would strip the brackets off that, v the cracks out, straighten it up and weld it. Lay another rail over that one and like to have at least an extra foot on each side of the damage. If you can span from crossmember to crossmember is better. If a rail can be found with flanges to go over the top and bottom is best but you can end the top at the bottom of the 5th wheel bracket if needed. Drill and bolt every hole in your existing frame and maybe some more.
A good job would out last the truck and a good certified pipe welder is can be a good investment.
New rails are best! Is the truck worth it?

 

Last rail I priced was for an 87 Autocar. It was the main or full length rail on a triple rail frame both sides were cracked. That was $5,000 twenty plus years ago.They said it came with all the holes drilled from the front of the bumper to the back of the cab. Sounds like time to buy a truck.

 

Don't forget to drill out the ends of the crack to make sure you stop it from spreading.

 

There are companies that'll build you a blank rail to any dimensions you want. Drilling isn't that hard as long as you take the time to lay everything out accurately.

FWIW I priced out 2 brand new undrilled rails from KW for my W900A and the were about $9k each IIRC. Ended up having blank rails made for about $50/foot.