These cars go through axles and bearings faster than they should, poor design for a car this heavy. 90% chance it is the axles and bearings, which you should replace. The quote may not have included those, but sounds like a fair price. Get the 31 spline axles and a new limited slip, your stocker is 28 spline and won't fit the 31 spline axles anyway unless it was already replaced by the previous owner.
Lifespeed
2004 Silver Birch Marauder 130K miles
Wilwood Aero6 F, NMDP R brakes, Penske 7500DA shocks, Hypercoil 600lbs F and Grand Marquis R air springs, Addco tubular sway bars, Metco control & Watts, 31 sp axles, Stainless Works cat-back exhaust, Lidio tune, American Racing AR883 9.3" 50mm F 10" 59mm R 20" wheels, Pirelli P-Zero 265/35-20, 275/40-20 tires
Coming soon: Livernois 5.0L stroker, ADTR/Vortech V2 intercooled SC, Silverfox trans and Circle D converter