where is the motor mounts located on 1998 Buick Century

engine moves a lot when put in gear

on subframe by crank pulley , between trans and subframe , between front of engine and subframe maybe some torque strut(s) up top. Visual observation , while someone is in the car shifting from drive to reverse may help isolate the bad one you're looking for if it's only one , but it's probably several of them .