I presume this is the front one at the axle end? And is similar to the Wrangler TJ, the bolt with a big ish T55?? head on it?
You will probably find there is quite a lot of lateral force on the bolt as it is a major player in keeping the axle in position. If I recall, I undid the chassis end first and that relieved the stresses at the axle end and the bolt then easily came out. Alternatively I’d mess about jacking up the chassis a bit to relieve the stress and see if you can do it that way. Either way, I don’t recall the bolt seized, it was just the sideways forces holding it in place.
Also, remember to torque it back up well when you reassemble it as there is one hell of a lot of stress on this one bolt.
