What should I check if 4 wheel drive isn't working on 03 F150 even though 4x4 light comes on?

I tried using my 4 wheel drive to pull my boat
trailer out of the water and the rear tires just dug
trenches!  I was Po'd because I took to a mechanic
and he said to put in 4 wheel drive so I turned knob
to 4 wheel high and the indicator came on and he
had me put it in reverse and he said it was working
because the differentials were fighting each other
in reverse but when I go forward nothing?  Any
suggestions?  They are talking at the shop they
need to open up the transfer case?  Wonder if it is
vacuum related?