Most likely its the alignment.
There are 2 things in the alignment that could be causing that, either the toe (the way the tires point the front of the vehicle) or the camber (the way the top of the tire leans in or out from the vehicle). Toe, on most vehicles, is the usual culprit, whereas on these trucks, camber does play a huge roll. Over time as the springs sag from age, it gives you some negative camber (the beams are like a huge lever, the fulcrum stays the same, whereas the end changes) which would wear the inside of the tire because more force is riding there. With the toe, instead of the tire rolling, it is being dragged a little, which is wearing that edge.
I would recomend taking it to a local alignment shop and having them check it out for you. As far as cost, it depends on what needs to be done and how long it may take.
If it is the camber, there is an eccentric (on these trucks, its about an inch tall with a hole all the way through it, but the hole is offset so as it spins, it will move the steering knuckle back to where it should be) that may need to be replaced, which can get quite pricy, depending on the shops labor rate (that is if a shop does it).
If it is the toe, an alignment will fix the problem if done by a qualified mechanic.
Now, Assuming all of this will fix your problem, the wear will not disppear immediately. it takes a long time (depending on how much wear is there) to get an uneven wear pattern out.
I hope this helps.
Cameron
Welcome Visitor! Please take a few seconds and Register
for our forum. Even if you don't want to post, you can still 'Like' and react to posts. 
)
