If the diff is locked, it'll bind. It stops the wheels from going different speeds. Much like the early autos have a fuse for FWD too ;D
Mine's a TT box in a BF5 Legacy so it's a different scenario to this WRX, but willing to bet it's a similar sort of issue.
First port of call: Call the AA and mention it to them. "The car you checked for us, rego _____, yeah did you notice that it has a massive skipping and binding steering problem?" ...see if they'll do anything for you.
If they're useless, and if no stellar answers come from here, there are a few decent mechanics around here that are either club sponsors or come highly recommended from happy club members. Really though, unless it was sold 'as is, where is', then there ought to be some sort of assurance that a mechanical problem like that won't show up.