Yes...ask me how I know...
I just did the driver seat on my 2001 XJ and snapped both front bolts. The bolts on the 2001 are into a rectangular channel that is welded to the floor. The only way to do new bolts if you snap the head of is to break the weld nut free with an air hammer, then run a new bolt through.
On the outside (closest to the door), it is fairly easy to fish the bolt though because when you trim away the carpet to access, there is a big round hole on the top (as well as an oval one on the backside). On the inside by the transmission hump there is only an oval on the back.
When you go to put the new bolts in, the issue is that you can't get a wrench or a socket through the oval holes to hold it tight. What I did was get new bolts and nuts so they would thread easily, put a finger though the oval hole to hold the bolt as tight as I could, then run the nut on aways. Once it has cleared about 1/4 inch of the threads, I grabbed the threads with a vice grip to hold the bolt tight while I tightened the nut with a wrench. One other issue is that the seat sits pretty low, so it is hard to get and arm under there to get a finger into the oval hole.
Last thing to warn about is that the weld nut and the end of the cracked off bolt faces down, so if you air hammer it loose, it may end up aiming downward and break though the floor pan. Ask me how I know...
Sounds like the OP's problem isn't broken welds or broken seat braces or brackets, but rather worn out foam padding. If you can find a good driver seat, just swap it and be done. Otherwise, I would guess the pax side seat foam would work. The bottom frame will not though. They are shaped differently and have different bracketry. Foam seems to be about the same dimention (although I have not taken the foam out of either).
Another option would be to pull some WJ seats from the yard and upgrade if you can find a good set (leather).