If you have traveled several times with no issues, then suddenly you have an issue, the problem lies with something you changed. You mentioned 2 things, tires, and what was loaded in the bed.
Load up the bed next time as you've had it before. If the problem goes away, from now on, you need to either physically load the bed more, or experiment with trailer loading for more tongue weight.
If the problem does not go away with the loading returned as previous, then you know the problem is the tires. Try adding more air, beyond what the truck sticker calls for, and closer to the max on the sidewall.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller