Those run the ATF-4, right? A mechanic pal told me that there was a little known issue of water contamination in thr trucks. The A/C drain is just above the trans dipstick and if the dipstick seal is compromised, the A/C condensation migrates into the trans. That could be the cause of the rusty areas inside. I'm sure that water mixed with ATF would result in a sludge of some sort.
