Swing/drift in while driving

I've had this car for 3 months now, (140,00miles)
lets just say big lesson learned my first time buying
a used car. Anyway I recently, yesterday, had to
spend all of my x-mas $$$ for my kids' presents on
getting my front control arms replaced/fixed & an
alignment. The mechanic @ ford says my car is safe
& will be great for a long time as long as I take care
of it. It wasn't until we got our snow/ice roads that I
noticed/felt that problem of pulling/jerking left &
right that had brought me to get my front fixed.
While driving it home after it being in their shop for
2 weeks, when I get to speeds 30+ mph, it my rear
end starts to drift/swing, to the point of pulling over
because I was so scared of drifting into other cars.
Before I bring it back & spend anymore $$ I don't
even have, what could be the cause of this?