the diff housing, and all that is the same. In fact, most of the rear drive toyotas share a lot of the same components out back. The supra for example shares a number of rear diff parts with us.
As for the suspension arms and such, that's all the same as well, or at least they are interchangeable. I do not know if there are any differences between the brakes or the suspension arms for sure, but I wouldn't think so.
For example... the rear member braces from Tom's and TRD all fit the IS300.
In terms of differentials, they're the same housings with different gears/lsds.
OEM is best.
02 IS300 E-Shift.