[84LX89GT]

Most likely i imagine it would be the lower control arm bushings in the front of the vehicle. Bushings are rubber insulators/flexible mounting points that tie moving parts of a vehicle to a fixture/frame. For example a lower control arm moves up and down when you hit bumps (tied to a "knuckle" and "spindle" which is tied to the wheel), to isolate the noise and harshness of movement and shock bushings are used. The springs help absorb and deflect road shocks and shock absorbers dampen the springs and help absorb the shock from a bump or just road irregularities. Most likely the rubber lower control arm bushings have aged and hardened, which causes them to crack and eventually fail. Failure would cause lots of noise and your car would be hard to control because there would be alot of slop between the moving suspension parts (lower control arm, spindle/knuckle assembly, and wheel moving excessively around the frame).

Bushings are a common replacement item in high mileage vehicle so I personally believe the cost is justified, plus your mustang may just ride better after you have it done.

Good luck,