Do you have a frame spreader handy?
Seriously, a LOT of Discos don't get their filters changed because of the difficulty in doing it. I've "heard" that someone managed to slide the pan horizontally far enough to get it to where they could change out the filter with the crossmember in place, but I've not seen such successfully done. You shouldn't have to remove the exhaust if you can get the crossmember to move enough to wiggle the pan out. Which is why I mentioned the spreader, like you use for body-work... basically a hydraulic jack, but with feet on both ends to use to push things apart (was also handy when putting the galvanized bulkhead back onto the chassis, as the galvanizing had drawn the lower bolting points in a bit towards each other). With it, you could get a hair's more clearance to get the crossmember to move a little more (I wouldn't take it all the way out), then continue on.... it might help a little to use a floor jack under the transfer case to get the transmission assembly to move up a little, to get you a bit more room, too.... we're not talking about loads of room here, just enough to jiggle the pan out, then back in.
Good luck....
-L
'72 SIII SW 88"
'60 SII 88" RHD