Try to check the rotation of the engine with the glow plugs removed so there is no compression. I would suggest that you spin it by hand if you can, then remove the glow plugs and then spin it by hand again to see if it is still the same force to turn. If it is still hard to turn that indicates that there is a significant drag from something in the engine (and that will be a rebuild of the engine itself probably).
You may also have some hydraulic fluid restriction somewhere so if you could remove anything hydraulic from the system (FEL, backhoe,…) and only use the simple hydraulic circuit that is part of the tractor that can show you if it is hydraulically induced problem.