Before you replace it, this is one of the few times I'd recommend an oil additive. It may just be dirty or sticky and the proper additive could possibly free it up. Seafoam is advertised to work for this. I would add it at an oil change, don't throw it into a crankcase full of dirty oil.
2003 F-150 XL RCSB 2wd 4.2 M5OD 3.55 LS