Welcome. $4000 with 84,000 miles -that'* outright theft.
I thought I'd gotten a deal in December, 03 when I bought my old 1999 SSEi with 142.000 miles for $5500.
What is the temp there in Connecticut? If it is below freezing the squeak might be the back window seal -I noticed in all 4 of the Bonneville'* (1992, 1994, 1998, and 1999) when it gets down below freezing the rear window seal will make a squeaking sound at almost every crack in the road -until the interior temp has been warmed up and warms the seal up.
Otherwise if it isn't that you might want to check the rear sway bar end-links. Mine were shot in my 1998 SSE when I bought it -I replaced them with standard Moog end-links from Auto Value ($13) and had them replaced in under 1/2 hr. They are a fairly common problem. If you want to spend an extra $4-7 get some Energy Suspension End-links -you'll not regret getting them.