3.7 rear axle ratio would tow better.... You also can beef up your 10 bolt rear end.
I didn't see if you are 2wd or 4wd... So you'd have to change your front gear ratio too.
At 5700 lbs you are fine with stock 10 bolt. You can replace the differential portion
of it with helical gear type diff and that will fix the weak point in the G80 Differential..
I bought all the parts to do mine last year (haven't had time to do it yet) , but in the
meantime I located a 9.5" rear diff with very low mileage at smokin hot deal..
Your stock 10 bolt axles are fine. they are not the weak point.
Just remember that your RPMs will climb some with the 3.7 gears if you are
going to be towing in 3rd gear.