2005 Silverado steering is wandering after changing tires

I have a 2005 chevy silverado 2500 hd crew cab with the duramax allison package. It has 42,000 miles. Well my problem is that I had the original duravis m773 265/75r16 tires until about 4,000 k ago and they where bald so I changed them to toyo open country 265/75r16 10 ply tires.
I started noticing a bad wander on the highway above 60 mph I then changed to michelin 10 ply and experienced the same problem so I went to the dealer they said I needed a tpc rated tire.
so they sold me a set of bridgestone dueler 10ply 265/75r16 and I have the same wandering.
the dealer had my car for two days and when I got it back they told me nothing felt wrong I then brought it to another dealer who had it for 4 days and they said they felt nothing wrong.
i've driven this truck since new and never had these problems the truck is now scary to drive.
i've been told that my shocks could be bad, make diff could be stuck in posi, my steering could be sloppy, and my wheel bearings could be on there way out but nobody knows what is accually the problem, and I dont want to replace things that are not bad! PLEASE HELP Thanks, Tom