I would be checking the belts out to start with, if they have a siny appearance on the inner V it would suggest to me that they have been slipping, however thats not always the case, tension on the belt would also be another area I would start on. In my eyes belts are relatively cheap/easy to adjust rather than trying to pull the back axles out to check on the clutches.
Im no greenfield expert though, there are some with more experience on here that will probably know whats going on without thinking about it
