Getting those fixed is easier than you may think...
To get the broken ones out, just use a punch and a BFH. That will break the weld that is holding the nut on. After that, use a wrench to fish a new nut in.
You can take it a step further and eliminate the need to deal with this again. If you have access to a stock rear XJ sway bar, you can use it as a bar-pin eliminator. The sway bar mounts to the u-bolts with a bracket, and that bracket is the perfect size to use as a top shock mount. Bolt it up there and bolt the shock to it....you will never have to deal with those rusted bolts again.
Sam