I traded in my 06' with almost 170k miles and brutal winters here with TONS of salt. While my doors and rear fenders were rotting my undercarriage looked fine. Just surface rust and not an ounce of heavy scale that was weakening anything. I bought a case of Fluid Film to spray my door and fender interiors this winter with to hopefully avoid body rot. But as for the undercarriage i would not be worried about it.
2011 Ram Big Horn 5.7 1500 4X4 QC. Air Bags, Bedslide, Mirror Tap/Blendmount, Kenwood DNX571HD, Alpine amps, JL Audio sub, Kicker upgrade, Back-up camera, RAM Lockbox, AMP Bedstep, Cabin filter mod, Rigid Dually flush Back up lights, V-leds license plate b/u light, SR-Q2 flush driving lights. Spyder LED Tail lights.