put the truck into neutral, remove the shift cable from the tranny linkage. the cable should be in perfect alignment with the linkage and slide in and out with ease. with them still disconnected, now put the shifter in 1st gear, and now the by hand shift the tranny into 1st gear, check that they are still in alighnment with each other. if that is ok remove the small cover on the driver side at the tailend of the tranny, its a little round cup with a hold down like a brake fluid resivoir has, slide the shift control selinoid out and clean it thouroughly and coat with new tranny fluid before reinstalling it in the tranny. if that dont work there may be a bigger problem to deal with. also welcome and good luck!