2005 Ford Escape XLT V6 Limited Edition (Rear Differential Issue)

I purchased my escape in March of 2012, The dealership replaced the rear differential with a used one.
15 days after the repair the differential started leaking and the axle boots were slinging grease as
well. I took it back to the dealership and they assured me everything was top notch. Obviously i'm
not mechanical in any way, but I do know this much the diff continued to leak, as the weeks passed
it started to feel like it had a pull and the rear end is making odd clanking and whining sounds, Now
its hard to even steer my vehicle and it seems to be locking up when driven slow or in stop and go
traffic. it also locks up pretty bad on any kind of descent. I called the dealership back and expressed
my concerns. They told me they only guarantee used parts for 90 Days. of course by the time I called
it was 134 days. does anyone have any knowledge as to how I could fix the problem and if the
dealership is responsible for fixing the issue being that it was never done right in the first place. Please
any information would be appreciated.