The driveline is the component that transfers the power of the engine directly to the wheels. It consists of a rear differential, a transfer case (on 4-wheel drive vehicles) and a front differential.
As you can imagine, the entire Driveline System requires lubrication to keep parts moving and to protect them from wear and tear. When gear lube is left in for too long, it no longer has the ability to fight friction and heat, however. The result is premature wear and eventually failure. Additionally, differential noise and "slip-sticking" on positrack systems can be...well, annoying.
A driveline service removes worn out gear oil and replaces it with new synthetic lubricants which protect better for longer.