as far as the springs go, get J1R springs all around, those are the "heavy duty" ones..
the ride is still excellent
for your airbag deal, dont know where you would go for that. the bags would mount to axle tube, directly above the spring perch, then onto the frame where your bumpstop is right now.
i'm not surprised that no one makes a "kit" for the yj, have a look at the gvwr and towing abilities of a yj, and you'll see why!
if you want to load it down, i'd have a look at a SMALL trailer, or a different vehicle for carrying all your stuff, they yj isnt made for it, and would be unsafe doing it IMO.
1998 XJ on 37's.JK Rubi axles with chromo shafts and ctm's, 4:1 transfer case, etc etc etc.
2003 Range Rover HSE, 4.4 Quad Cam.
1998 Range Rover with a 6.0 LSx
1994 YJ, 4.3/700R4/OME SOA, etc
Western Star 4900EX 4 axle.
4.3/700r4/new frame buildup (YJ)....