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For a 4BT your axles will be fine provided you are doing any major towing and you are running 33's or smaller. If you said 6BT, then you need bigger axles front and rear.
For the 4BT, you could go 3/4 tons, it would not hurt. D44HD front and like a D60 or 14FF in back. But the 4BT does not make all that much more power than a typical V8.