Final answer:
A multi-speed rear axle and auxiliary transmission are two components in a vehicle's drivetrain that help improve performance and efficiency.
Step-by-step explanation:
A multi-speed rear axle is a component in a vehicle's drivetrain that allows for multiple gear ratios to be used for the rear wheels. This improves the vehicle's performance and fuel efficiency by optimizing power delivery to the wheels based on the driving conditions. A auxiliary transmission is an additional transmission system that works in conjunction with the main transmission to provide extra gear ratios. It is commonly used in heavy-duty vehicles like trucks to improve towing capacity and overall performance.