I had the same thing happen to my 96 dakota except it was on the passenger side. It rotted all the way through right before the leaf shackle. That bend is the weakest spot on those frames. Someone told me that there was a batch of 96 dakotas that the frames were not painted properly at the Chrysler factory. Apparently when they painted these frames there was moisture on them before the paint was applied so the paint never stuck. Also, he said that Chrysler knew about this but didn't think that it was worth repainting this batch of frames. They figured that the truck would be off the road and in the scrap yard before this would become a problem.

THe way I fixed it was by welding in 2 peices of 1/4in angle iron (one on the top and one on the bottom of the channel). You could also use 3/16 because that is what the frame is made out of to begin with but i couldn't find any so i used 1/4. I used a 200 amp arc welder. I think I had the amperage on like 150 or 120 i don't really remember. I had to cut and bend and then weld the angle iron in 2 places to follow the curvature of the frame. Also I filled the 2 or 3in gap in between the two peices of angle iron in a few places for more strength. I drilled many holes in the frame and plug welded many spots for more strength and then bead welded anywhere else I thought was important. Next time there is another problem with the frame its gonna get a new one. Its really a shame because its such a nice truck . There is no rust or dents anywhere on the cab or bed. The original red paint even looks great. If you looked at the truck from 2 feet away you would think this thing was always garaged and only driven on the weekends in nice weather! The body is perfect, the frame is trash.

 

im slowly but surely working on the fram in myne aswell. my is rotten at the trans cross member on each side. i used a air hammer and chisled out what was bad then cleaned up around it with cutting wheels (the idea was if it bent to easily or broke away it was bad and i was trying to save what i could) and then welded in about 3 inches past each side of the are with 1/4 in angle iron. i have done the drivers side outer and lower channels where it was worst. im considering trying to go to the local pull a part and cut out a section of frame (have to do it by hand they wont let my use power cutting tools or a torch) and the weld in angle iron on the insides of the rails and making a nice pretty butt weld with a band of flat stock over it. will it be over kill? yes. will i sleep good knowing the next day i may put 1500 pounds or so in the bed? like a baby.

the only other option is the cut out the lower halves of the rails in that area and build it back with iron. doing that means fun work getting the cross member right.
im still considering that option.
i can promise you though that you will feel a difference in how it drives. i fealt a huge difference. it stiffend it up considerably.

as someone else said though. i am just amazed that the truck could carry it's own weight like that.