they are part of what gives an MJ a 2k load rating. from the factory the truck would also have had tougher shocks, front springs, wheels and rear axle. In theory it'll ride a bit stiffer than a regular MJ, but it's not all that bad. oh, and the leaf packs came in 2 varieties over the years, 4+1 and 3+2.
be cautious. time/abuse can be a killer to springs. just because they were rated for 2k 30 years ago doesn't necessarily mean they are capable today.