Transmissions will tow in OD just fine, provided you have enough holding power in the clutch packs or clutch to apply power and not slip depending on your final drive ratio and GCVW.
That said the biggest hiccup with the 60e is it's car like .70:1 OD. That combined with limited 3-4 clutch pack holding power results in accelerated wear and possible slippage even locked in. Even going to a Z-pack to increase the surface area and effective holding power of the 3-4 pack doesn't alleviate the steep OD ratio which makes the shift out of lockup nearly all the time and even downshift regularly. You get the heat from the converter in non-lockup and the wear on the clutches from the downshifting.
You could regear the axle a bit deeper to make up for the OD but it's the opposite of what you're trying to do in lower cruise RPM.
IMO for your situation, run in OD on open flat ground and just run in 3rd anytime you need more power/compression braking.
I too run my manual (G56) in OD grossing 20klbs, but it's a manual and my OD is .79:1.