Swap rear ends or transmission?

Just a thought:
If your drivers call it a 10 speed, and in fact drive it like a 10 speed they may also be very abusive/brutal to get going in soft ground.

Even though the gear reduction is not as much (less torque out of the tranmission, in theory) the abuse may actually be far worse and be part of the problem.
Using the deep reduction gears and being more gentle with clutch and accelerator may give better performance off-road with less breakage.

 

I think you need to give more details on the kind of work you do. Is it dig out work? Where the ground is really soft? Or is it quarry work where the trucks and unloads mainly on hardpacked ground? Paving jobs? These details have a big impact on transmission choice. How much weight are you grossing? Lift axles or not?

 

Imo keep the 15 it's a little more rugged "harder to tear up" than a 18 for random drivers.

Put your money in some .390 gears good balance off road with decent hwy speeds. Friends truck is set up with 390 gears tall 24.5 and 15spd. @1700 at 70mph I believe.
I

 

70-90k on 40k rears? You must have a lift axle or 2 on this truck?

 

So that poor truck is definitely overloaded. 78k max assuming 12k steer axle and 86k if its got a 20k steer axle. I'm suprised 40k rears and the AG100 lasted this long.