I have a 99 ECLB K2500 w/8600 lb JB7 brakes (2.5”) drum brakes on the 14 bolt 9.5 SF rear axle w/o Gov lock (G80) & 3.73 (GT4), I have the 5.7L, and 4L80 also.
I believe the deciding factor on FF was powertrain combos. In the K2500, 5.7L got the SF, you needed either the 6.5 TD or 7.4L gas motor to get the FF rear axle. C/K3500 automatically got FFs even with 5.7L.
I think 2500 suburban got were the same as far as rear axle being dependent on what engine. I think the burbs FF rear came with slip on drums and hydroboost brake, JH7? (3.5”), not vacuum boosted (2.5”) JB7 brakes.