I have a wrecked 2006 wrx that i will be using as a donor car for an engine swap in my RS. I will also be using a ton of parts of the wrx being as my car is not in as good of shape as the wrx

i have someone interested in purchasing the rear knuckles (between axle and brakes), but i was thinking if i should just keep them and put them on my car?

next thing would be, would i need them if i will be putting in the wrx rear diff and the rear axles?

I'd swap as much of the new parts as I could. Sell off the old junk instead.