My previous Impreza had a squeaky back end too !
As has been said it's probably the bushes and in my experience , normally the anti roll bar bushes . They need to be removed , the bushes cleaned inside together with the area of the roll bar that the bush locates on , and the whole lot put back together lubricated with white grease . On my current Impreza I also wrapped some PTFE plumbers tape around the anti-roll bar before also applying the white grease .
Been squeak free so far .
Probably start squeaking tomorrow now ....
John H