Not sure why but you'll find that this is characteristic of these years drum brake trucks. Mine also does it but only on cold/wet days. But I've had GM and Ford drum brake trucks that have never done it. And its only the first stab of the brakes too. The rest of the day they're fine. Its always good to check for leaking cylinders, broken springs, maladjustments, worn parts, etc, but as you noticed, its common to find nothing wrong. Almost like its brake dust build up or the way the OEM shoes react to the thin layer of rust that builds up overnight on those cold/wet days.
'01 Dodge Ram 2500 4x4 5spd Sport, EZ, Straight piped, aFe intake, Rancho 9000x's, 285's, Pac Brake, 2 lo kit, Dakota FP and Isspro boost/EGT gauges on the pillar, Raptor with 1/2" big line kit.