I apologize if I am repeating any advice already given but a pull is almost always camber/caster/toe (I do a lot of alignments at my job). I understand that you have already had an alignment, or more than a few.
I'm assuming you are not the original owner...
The rear axle alignment is not adjustable so you can put that to bed but it is worth taking a flashlight and inspecting every inch of the frame for mushrooms or other bends, repairs and welds, or anything generally out of place.
The fact that you have a brand new front end and still have alignment issues is alarming, when a vehicle is in an accident they never align quite right, even if the computer tells you it is.
You may have unknowingly bought a burb with a compromised frame. If you can't find any damage, its worth paying a shop a small fee to inspect your frame/chassis.
There are many elements to inspect before the frame but it seems you have covered all bases.
Nick