Power Steering - Pump or Box?

On my '96 XJ, 4.0L, AW4, Np231; my steering is very hard when I first start it, and sometimes on the trail when the engine is idling. It feels like it does not have power steering. What should I replace first - The pump or the gearbox?

Cheapest fix would be to try and swap the fluid. I've heard the best to use was the Mopar kind from the dealership.

Fluid.

Then pump.

Rev