I'm so happy for you both. I wonder how much torque that inline puts out? Typically inline motors put out decent torque and lug very well where similar displacement v6's need rpm's to make any power.
My engine has the same HP specs as yours but with a 1000lb higher curb weight and likely much taller gearing and I run 20's instead of the factory 17's. I also had my truck and trailer loaded significantly higher than I would normally had I just been hauling skis. I had to put 45lbs in my airbags to get the truck back to level and 50lbs in the trailer tires and they were still bulging. Considering the towing capacity of the truck is only 3050lbs, I'd say I was pretty close if not well over it. I guess I'm just spoiled from all the years I owned BB 460's, Cummins and V10's. If it means anything, I always thought the 4.7 V8 was a bit of a dog at towing too.
Getting back to some of the comments above, no need to swap axles, just the ring and pinion gears in the rear end. Not sure on this exact axle but on my bigger trucks, you could swap the entire carrier in a couple hours and never even have to worry about resetting backlash. Available gear sets are 3.55 and 3.92. I believe mine has the 3.92's.