How to replace an o-ring on top of the steering gear box.

I looked in our service manual and it is easiest to remove the steering box. In fact that is what the instructions say, you might try to do it in the truck, but it would take longer than removing it. There are a lot of alignments that must be kept in place, so mark everything with a grease pencil before you remove it. If the bearing is bad it is definately going to be a hard job. If it seems that the bearing has failed you should take it to a repair shop as the gears and the mesh spacing must be measured as the gears cannot be serviced and you may have to get a used or new steering box.