[Brad54]

I can't answer yes or no on whether it was available from the factory, but it would NOT be in an original automatic-equipped truck that came without locking front hubs. From what I've seen, automatic trucks always had fully-engaged front hubs--they'd make short work of a posi unit up front.
Now, if it was a manual trans, with manually-locking hubs, there's no reason it couldn't have a posi unit it in, as driving down the road with the front hubs disengaged wouldn't be spinning the posi unit and wearing out the clutches in turns and corners.

-Brad