I once had a similar problem with an '87 Suburban. Here in Oregon the grass can be so slick that you won't get any traction even though you aren't sinking in. Nothing wrong with the truck, just street tires that filled all the tread with mud and grass and they became racing slicks. I was on a hill behind the house. By the time I got it back to the asphalt driveway I had another standard towdruck stuck about 75 feet above me and a Big Bertha tow truck that had to snatch onto the base of a telephone pole to get unstuck before he could winch the towtruck and my truck up the hill. I told him not to drive out on the grass and he just laughed and told me how tough his truck was. An hour later he was a lot more humble, so don't feel bad about spinning your wheels. there is probably nothing wrong with your truck that some gravel, sand, pieces of wood, etc. won't cure.
