While Looking At The Front Of My Truck.....
 

What might need to be replaced if my front tires look like they are bowing inward at the top...

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Its not really angled like that above but i can tell there is a slight angle

Get the alignment checked first. There are a few things that can cause this like the upper control arms being bent or bushings wore out but it's possible to fix with just an alignment.

I am not saying that I know what the problem is exactly, though I would check the alignment, could be a simple adjustment issue-

Definately check the alignment first, sounds like the toe in is slightly out of adjustment. Sixpack

if they are leaning in like that it's not the toe it's the camber. you have too much negative camber. possably being upper control arms but if it's a 7.3 4x4 then it should be a solid axle IIRC and i'm not sure how those really work with frontend adjustments.....

if its leaning that bad go for ball joints first, then your probably looking at wheel bearings possibly, then springs, could also need a polyurethane bushing kit along with it, worth the money, alignment wont fix that, no way an alignment can, ford TTB trucks are notorious for doing this, ive worked on several that were horrible.

IF it is a TTB front end the Camber bushings are bad for wearing. I had one break apart on my old Ford and fall out.

I am assuming its a f250 with ttb the springs on those things do wear out or like he said above it could be the camber bushings

I didnt think leaf sprung front ends have control arms. typically this is a ball joint or camber bushing issue

I say get out while the gettings good. Lose that TTB and go for a straight axle. A properly functioning TTB is a decent set up but I'd guess for what you'll spend getting this corrected you could possibly get a straight axle underneath that thing if you do it all yourself.