You can only adjust the steering gear so far. As the gears wear internally, they will wear close to the "straight ahead" position most, while the "full turn" position will not show a lot of wear. If you adjust the gear lash for a worn out, "straight ahead" its likely the gears will bind at full-turn.
Without seeing the truck its difficult to be certain of anything, but the steering gears do wear out, and they cannot be adjusted into infinity.
Current truck: 2002 PSD 4x4, CC, XLT, F350 dually, 4.10 gears. Air bags, 6.0 trans cooler, Fleetrite ELC, ISSPRO gauges (boost, EGT, trans, fuel pressure), PHP Hydra, full EBPV delete (pedestal & outlet), CNC Fab HP & HPX lines, Riff Raff FRX, Riff Raff IC boots, CCV mod w/catchcan, Reese Titan, AIS, ITP pre-pump filter kit, in-tank mods, Redhead steering box.
Old truck: 2000 PSD 4x4, ext cab, XLT, F-350, auto, V-code springs, air bags, coolant filter, 203 thermostat, CCV mod, Fleetrite ELC, DB Alternator, DB high-torque starter, Tru-cool Max-cool trans cooler, B&M trans temp gauge, 47 gallon Titan tank, ITP pre-pump filter kit.