Jeep. Jeep? Jeep! Jeep.
Join Date: Jun 2004
Location: North Caldwell, NJ
Only real way to fix rust is to cut out the rusty stuff, weld in new. The frames rot out like that because of poor drainage, and buildup of mud and salt inside. Mine had the same thing happen, I just found a guy with a welder who gave me a good deal on the work. He repaired the front and rear with thicker than factory metal, I shouldn't have any trouble for a long time. If you see a shop that specalizes in Jeeps, they probably did the same repair in the past, and will know how to do it again.
Best bet would be to find someone who's parting out a Jeep and see if they could cut those frame sections out, you'll at least have a good clean template if you want to cut and weld in thicker sections.
Most of the cost is going to be in labor, so if it's something you could do, then by all means try to tackle the project yourself.
Also, check the front of your frame around the steering box as others have said. That's another spot where nasty rust happens.
1989 Jeep Wrangler - Daily Driver and project - Red Jeep Club #216, NRA Jeep Club #5
1967 Chevelle SS396 - Main Project <--454, TH400, 4.10's, FAST
1982 Caprice Classic - Another project <-- 350/700R4, 4 wheel discs
1979 Honda Civic CVCC - Yet another project <--goal of 60 mpg