It's the main tubular rear subframe you need to check carefully, and protect as best you can - part 1 in the diagram below.
If you get serious corrosion here, you can probably write the car off, no matter how good the rest of it is, at least unless someone decides to start producing an aftermarket replacement. OEM ones have been unavailable in the UK for some time now, and even when they were, cost north of £2k.
IM axles do aftermarket trailing arms at reasonable cost (the bits you've just had replaced), but not the main subframe.
The front subframe is also unique to this model, and corrosion in this part has recently written off another 4x4, so it's worth checking this as well.