If it's 4wd its a W250, not D250.
The easy stuff you can check and replace on your own:
Drag link, goes from the pitman arm on the steering gear box to the drivers side steering knuckle. Grab the center of the bar and try to wobble it. If it moves real easy, replace it.
Tie rod ends, thread into either end of the center link and attach the steering knuckles together. Same procedure as the drag link.
Pitman arm, check and make sure it's tight on the shaft coming out of the gearbox.
Steering shaft/coupler, grab the shaft and rotate it side to side, if theres slop inside of the coupler, replace the shaft and coupler with a borgeson unit.
Steering gear, provided the shaft is tight, move the shaft again with your hand to make sure the pitman arm moves right along with it. If there's slop, replace the gear, or tighten the adjusting screw on it.
Do your standard check on the wheel bearings and king pins. Jack the front end up, grab the tire, and try to wobble it back and forth vertically. If it moves, either your king pins are bad or your bearings. You can figure out which by watching the knuckle as you do this. Bearings are an easy fix... king pins will have you hitting the bottle early.
The plate and mounting bolts for the steering gear can loosen up, as well and the plate can crack. This you'll need the truck running to see. Have someone turn the wheel and make sure the gear stays put.
Some of the less likely things could be loose u-bolts, incorrect caster if you put a lift on, or some other wierd stuff like that. Some stuff won't show up until you watch it with the truck running with someone turning the wheel though. You can't replicate the force that the hydraulic unit can put on the steering system.
The biggest cuplrit is usually the coupler on the steering shaft, followed close by the drag link and tie rod ends. Fixing these three things will make your steering feel like new usually. If any of the aformentioned parts are bad, they may refuse to do an alignment. All of these fixes you can do on your own, with the exception of maybe the king pins depending on your patience, inginuity, and most of all your aresenal of tools. Good luck.