I had kind of the same problem with my trans. Taking off felt like it was sliping,but it was not it was taking off in second or 3rd. It was some kind of pressure siloniod and if you stop it should go to a 0 presure making it go back into 1st gear. The way I found out is drive your truck and come to a stop and then go again. If it feels like it slipping try it again but this time stop and manualy put it into gear and than take off.If it takes off right away you might have the same problem