[Richard]

The fluid is ATF. The reservoir is in the engine compartment passenger side. It is the same system that powers the hydraulic fan.

That gearbox is HEAVY. Almost any truck shop should be able to replace it for you. Weller is a well know reman company.

However, the steering gear boxes can have a common exterior case but different ratios and internals that affect the steering characteristics, so a rebuilt one may not be exactly like yours. You will have to find the tag on the box to get the specifics. If you can’t match the exact model, then sending yours to Weller for reman may be an option.