Towing with a 4.7L you're going to want at least 4.10s. Also, I don't believe 3.73s exist for the 9.25" rear axle, that's a gear option for the HD trucks. If they did, it's only a 94 RPM difference at 70 MPH. The 4.7L up to 2007 only has 235 HP stock. It needs a lot more gear to get it moving and maintain highway speeds, even more so when towing. I had an 04 Quad Cab with 3.92s and stock size 33" tires and hated it the first time I hitched up my new boat. I ended up re-gearing to 4.56s and it helped a lot, but still not great with the auto transmission. It soon got traded on a Hemi Ram. That NV3500 manual trans will help some...
With the NV3500, 4.10s would be a good choice since you only have one overdrive gear. Depending on the weight you're towing and the frequency, 4.56s might be better. Here's the ratios of that trans if you're interested.
FIRST 4.02
SECOND 2.32
THIRD 1.40
FOURTH 1.00
FIFTH 0.73
REVERSE 3.55
You have the 9.25" corporate rear axle. Could have LSD (limited slip/anti spin), it was an option. Need to pull the build sheet to confirm. The 8.25" axle is the front axle in a 4x4. You also don't need a whole new axle, just the gears and install kit, then find a good installer that has experience with Dodge axles. Install can run anywhere from $250-500 depending on the area.
Assuming you have stock 20" wheels with 275/60/20 (33") tires, here's your RPMs at 70 MPH in 5th gear.
3.55: 1847
3.92: 2040
4.10: 2133
4.56: 2373
If you have stock 17" wheels with 265/70/17 (31.6") tires, here's your RPMs at 70 MPH in 5th gear.
3.55: 1929
3.92: 2027
4.10: 2228
4.56: 2478
Lastly, speedo recalibration is not required with a gear swap. The tone ring is bolted to the ring gear so a gear change doesn't have an effect.