How old are they? Original to you '08 F150 would be ~ 6 years, that would be long enough for me regardless of mileage or tread depth. Age is a killer on tires. I suspect the sidewall cracking is from UV damage.
Just this week I had my first tire issue, admittedly they were low quality tires that the dealer put on while it was on there lot that I should have replaced immediately but I was trying to save a few bucks. While towing in the mountains I had the tread separate on my right rear. Not a fun experience -- that said if you have doubts on yours have them replaced before the trip.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)