What will also help avoid any potential final drive problems with straddle duals is to just not over-fill the grain tank. Skip the extensions this year. Straddle duals and a Powering the rear wheels will really help. Another option is going to the super wide, super low pressure single wheels.

Powered rear wheels particularly help in steering. They tend to climb out of ruts when you try to turn rather than having to be pulled up the front tires' berms. Maybe you could find a used Mud hog setup for the 1680. There were a lot of those combines built and may one in a yard somewhere that some one would sell the powered rear axle off of.

In any case, keeping the grain tank weight down a bit in a wet year helps. jmho & experience.

Jim at Dawn