Use your judgement and you'll be fine....there's no black/white answers to these questions really. I used OD pulling my 21' boat in flat, no-wind situations where I was barely on the gas. And that was over double the weight.
If it's constantly shifting in and out, or locking/unlocking because of conditions, simpy knock it down a gear....towing or not. That's when you're straining things, not simply because you're towing something.