Roll down the window, Or push it down. Open your tailgate. Push the left side striker latch closed, with the access panel out, you can look in, and see what's going on.
It sounds like the bushings in your gear head are gone. This is a common problem with old window motors. The worm gear spins, but the plastic gear, that turns the steel gear, just sits and spins.
If this is the case, take your motor out, there will be a phillips head ***** holding the gear cover on, take it off, and pull the steel gear out of the plastic gear. There should be 3 round plastic bushings about 1/2" in diameter under it. You will probably find at least one of these is powder. You can get the kit at your local autoparts for about $15.00, or you can make your own, possibly free. I will tell you how, if this is the problem.
Let us know how it goes.