The first important parts to get damaged from rust are the pinch welds and brake lines. The amount of rust on that diff and tow frame is trivial considering how f*ckn thick that frame steel is. That surface rust is completely normal for the age.

What you should look at as far as rust damage are pinch welds, brake/fuel/cooler lines, rocker panels and wheel wells inside and out. If you see the paint bubbling on rockers/wheel wells, it's rusted from the inside out, or if rust has already reared its ugly head there, it will only get worse, and quicker than you think. Its only sheet metal.

But rocker and wheel well paint jobs are just aesthetics.won't hurt the drivability of the car, just the resale value when you own it for years and that little nick turned in to a hole in the body.

Alas, such is the struggle of living in the north. These things are unavoidable after a car hits a certain age, given standard treatment. That age just happens to be 10 years, plus or minus three for amount of care.