I've dealt with underbody rust before, used Rust Bullet & 3M rubber coating over all of it, and also some new sheet metal where it was needed. But - this seems bad because it's right behind the shackle on the LR - is this a major concern? I want to patch it, but is the rear shackle going to break off? How should i properly fix this?

 

Looks sketchy...

 

looks like a normal northern Jeep to me... where are you located? wheres the Jeep from?

 

If it does you will be the first to know, lift the carpet, see what it looks like from above.

 

LR? I'd definatly dropand remove the gas tank to fix than. Cut your way in, weld your way out. Use some 16 or 14 ga steel, better than that was original. Pull up your interior carpet and pull the tail light lense out so no fires or damage for spatter. Disconnect the batter prior to any welding.
Have an extinguisher ready when welding and someone on fire watch, you can't see what else is burning when your helmet is on.
You can fix that for sure, no problem, plan on it to take some time. I'd coat with a few layers of rust paint after, I never recommend any of the ashpalt coatings, less effective than good rust paint and future problems. Added a pic to show the extent of what can be repaired with motivation, mor pics in the album. Go for it.

 

Jeep's from NJ, I'm in eastern PA.

 

Anyway to do it from underneath? I don't think the holes are under the carpet, more inbetween the body & plastic. Could i do it from underneath?

 

You can definately do it from underneath. I'd cut some 16g, no lighter than 18ga, to fit. Weld it in. It will be overhead welding so don't get burned by falling metal. You can do it tank in if you are absolutely sure your tank is in great shape, no leaks, but for sure disconnect the battery. Clamp the welder ground as close to the point of welding as possible and remove as much surface rust as possible prior to welding. I suggest the paint over after all done as you can weld thru the paint again in the future whereas not so easy with other coatings. Do pull back the carpet if you think heat will be in the floor. Fold it back down when all is done and cooled.

 

Alright, i'll have to give it a whirl, I've got a friend that does professional welding (small racecar chassis etc.) so i can recruit him if i need any help. Gas tank should be good, but i'll be careful with it to be safe.

btw- are you on FacePunch? I recognize the "peep"