It'll depend on model year (because they replaced the old 4 speed auto transmission with a 6 speed transmission), engine (I believe you could get a 5.3L, 6.0L and 8.1L in Suburbans), and the rear axle ratio also has a big effect (3.23, 3.42, 3.73). Higher numbers pull better than lower numbers with axle ratio.
As an example, my 2014 Ram 2500/Hemi was rated at 15,000 lbs with a 4.10 rear, but if it had a 3.73 it was 12,000 lbs. That's a 3000 lb difference just based on axle ratio.