1997 Mercury Sable Suspension/alignment

I bounced off a center wall a few months ago to avoid an accident, since then I have put 2 new rear stuts, new tie rod ends in the front, new lower ball joint on front driver side, 4 new tires and a 4 wheel alignment, and after that it still seems very floaty in the rear and wants to fishtail on icey/snowy roads, I just can't figure out why its so loose in the ass end and I crawled under and tugged on the sway bar and that seemed solid, any suggestions please I have put a lot of money into this and want it to be safe to drive my kids around in.