mk 6 winding rear axle

[ukbertie]

New hear so bear with me , i have a 2.5 3 way crewcab tipper year 2000 , i have noticed that she doesent pull as well as she used to and a grumbling/WINDING noise is comming from the back axle or drive shaft , do i have to top up with oil ? if so how or is it off to the garage for a replacement , i know shes an old van but ex council and im only second owner with electric windows fantastic heater and new engine 40,000 miles ago and shes a 3way tipper
any help would be appreciated