Corrosion between frame and rear crossmember - 98 Trooper 4WD

I changed my tranny fluid last week - thanks to all the good info from you guys, it went without a hitch. At nearly 60K, the fluid hadn't been changed
since new, and it still looked good. There was quite a ferrous "bloom" on the magnet, but nothing that didn't look out of place on it. I used an aftermarket filter from NAPA ($39) vice the $95 filter and $19 gasket from Isuzu, and the old and the new were identical. Took nearly 6 quarts of fluid by the time I was done, and of course, some of it was on me and the floor. I also used the OEM rubber-insert gasket/washer but Isuzu recommended that I add a large copper washer on top to ensure a good seal - price for all 4 washers was only $5.
However, due to a friend who did this before me suggesting it, I removed the crossmember to make getting to the pan easier. I was quite surprised to see the amount of corrosion between the frame mounts and the crossmember - nearly the entire face of the crossmember was exfoliating and allowed me to chip out several layers, adding up to nearly 1/8" in metal loss by the time I was done. Keep in mind this is on a 1998, that to my knowlege has NOT been immersed in salt water. The rest of the metal surrounding the mating surface seemed to be pretty good - no sign of corrosion at all. It was just in the mating area - leading me to believe there was some kind of bi-metallic corrosion there, but both parts appear to be made out of the same steel.
I didn't have a lot of time to analyze this, so I chipped, wirebrushed, and bar-rusted the surfaces and shot a coat of rustoleum primer and paint on the mating surfaces and put it back together. Just seems strange to me that I've taken apart plenty of 20 and 30 year old American cars and never saw this kind of corrosion before.... anyone seen this before on theirs?
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