Why does fluid leak near rear axle only when shifting in and out of four wheel drive?

it cold be teh driveshaft is moving to much due to worn parts in axle that are just now starting to expose themselves or it cold be a seal going bad. You have to find out wher ethe leak is coming from , the axle itself or something else.

Thank you. I just don't understand why it would only do it in four wheel drive and not all the time since the rear axle is always engaged. I could understand if it was coming from the front axle. Any further explanation you could provide for my understanding would be greatly appreciated. Thank you for all your help and guidance.

you really need to find out if it's actually coming from the axle or soemthing else. it may be a coincidence as well with the mileage on the truck it may have started leaking and you only noticed it when you put it in 4 wheel drive.