How does it work with cold oil? If it works somewhat with cold oil, and gets progressively worse as the oil warms up, you likely have either a worn pump and/or blown seals in the pump internally. Both are accessed through the pump mounting side cover on the front RH side of the rear axle housing.
On the other hand, if you have nothing even with cold oil, you most likely have a stripped out PTO drive shaft internal to the transmission. This shaft drives the PTO clutch as well as the hydraulic pump. This shaft can only be replaced with a double split, as the hub in the flywheel will also need to be replaced. If the PTO still drives a load, discount this as a possibility.