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.The Cherylkee....
2003 Range Rover HSE, 4.4 Quad Cam.
1998 Range Rover 4.6 HSE
2000 4.7 4WD WJ with a rebuilt motor.
4.3/700r4/new frame buildup (YJ)....
Years to gain the hands on experience, and offer it freely.
Minutes to gain the knowledge from the internet, and quote it just as freely.