Work on stuff long enough and you eventually develop many techniques for removing stuck fasteners.
Always go to the blue wrench first. Heat works wonders. If the heads are still good, be sure to use the proper wrench. You can link two wrenches together to get additional leverage too.
If you have a stripped head then your options are vice grips or sometimes a Metwrench or similar wrench that puts pressure on the flats instead of the corners will work.
If you can't get a wrench or vice grips to budge it then it may be drill time. Use a good center punch and drill the fastener completely through to accept a tapered square bolt extractor. Drive that thing in tight and carefully wrench it out. I would heat it again as well to assist you.
Working around the driveshaft flange may make some of these methods implausible. If you have no other option you can always just grind the heads off to get the driveshaft out. Then you can work on the flange unobstructed, or even remove the flange and work on it on the bench. You'll have to re-set the pinion bearing preload when you put it back together, but that isn't a big deal. I can walk you through it if you need.
'06 F250 4x4 - 5" Flo Pro exhaust, SB Filter intake, Accufab elbow, Edge Evolution (monitoring only), SCT w/ ID custom tune, FASS 195 pump, Gillette Diesel EGR cooler delete, Sinister Diesel coolant filter, ELC coolant, updated turbo drain tube/oil feed line/STC fitting/oil cooler, ARPs w/OEM HGs, Elite coolant lines, ITP RR fuel system, Warren Diesel 175cc injectors, Powermax, BD CCV, FICM.com FICM w/ ID tune, Elite UP, BPD water pump - shooting for the 12s!
'02 WRX - Outback rear disc swap, EBC green pads, pillar vane front rotors, TXS UP/TBE/TMIC, Perrin LW crank pulley, PPG billet steel shift forks, ACT Streetlite flywheel & clutch, K&N filter, STi Group N motor/trans mounts, TiC/Kartboy rear diff mounts/subframe lock bolts/outrigger stiffeners, Kartboy SS & all shifter bushings, custom PDX tune for Cobb AP - went 14.1 on a terrible 60ft before most of these mods; shooting for 13.50s