rear beam bushes would be the first port of call, just jack the car up and see if the beam bushes are shot or not, very very simple check

also, when you changed stuff on the front, did you do up the nuts and bolts on the arb and inner tca's while it was on the deck or while it was in the air?

you need to do them up when it's on the deck with normal weight on them so that they are located properly when they are driven, if you have tightened everything up in the air then the 3 or 4 inches worth of movement from when they are on the deck winds up the bushes and this can also give you issues