Why would my 1999 Toyota four runner be hard to steer?

If this came on recently then I would say one of several things. Belt cam off or broke from the Power Steering unit. The fluid is low in the unit, which could have a leak somewhere. The power steering unit is bad.

If this is just slightly hard to turn then it could be an indication of low air pressure in the tires.

If all this is OK then you could have a linkage problem under the vehicle. For that take it to an alignment shop and have it checked. Having the linkage break when making a turn is very inconvenient if not dangerous.