I had the same issues with my '96 2dr Tahoe, replaced all the front end components and the pump (Delco) & gearbox (NAPA) - hated the new gearbox so ponied up the cash for one from RedHead - that made a difference, but still not the greatest. One thing that helped mine early on was to replace the rag joint that connects the steering shaft to the gearbox coupler, they do wear out. There is a cheap kit for this. Some guys even eliminate it and install a u-joint style shaft, but I haven't tried that yet.
There is some info on drilling out the orifice and/or adjusting the spring for the pressure release valve in the pump, but do this carefully.
There is info out there about adjusting the over-center adjustment on the top of the gearbox. The right weay to do this is to remove the gearbox and also adjust the bearing preload. Just doing the over-center adjustment in the truck can cause binding near the center of the gearbox travel, and usally isn't the problem.