This is one of those , a picture is worth a thousand words , deals .
I would suggest going to an alinement shop , ask or look for one of those posters on the wall that show tire wear patterns , it will match pictures with problems .
Also most shops will give an free est.
Then post what you find out .
The reason I say these is that are many wear patterns , and many of those can have multiple causes , so whats happing is to answer your ? would require a lot of info that you may not have [ picture ] .
How many miles on your truck ? do you drive on bad roads ?