The 1999 and 2000 trucks had flex and oscillation problems in the front end. As I recall it was a second harmonic induced by specific combinations of speed and road conditions. The problem was bad enough that GM added braces to the front and trans crossmembers but I'm not sure what the differences were between 2WD and 4WD trucks. The hydroforming process for making frame members was introduced on the GMT800 trucks, and I think it took them a year of real-world feedback to perfect it.
Back to the rust though, the gauge material in these frame is light enough that rust can destroy them. Cracks are easy to see in rusted steel exposed to weather like that. Look especially at the seems/welds. Personally I would get underneath it with a bright LED light and start inspecting - it could be a deathtrap.