2006 Explorer 4.6 v8 has no power to tow up mountains?

My older (97) 4.6 definitely didn't have enough pluck to tow our 6000ish lb trailer through the mountains until I upgraded the rear end from 3.55 to 4.10. After that I had to get used to the high rpms, but it did the job reasonably well. On edit, I should have added that my setup had the old 4 speed auto. Dunno if Ford had upgraded to 6 speed by the 2006 model years