What's wrong with my truck

Manual shift or automatic? You said park so i'll assume automatic . First step check your fluid level. With the engine cold pull the transmission dipstick. Check for fluid, it should be pinkish, not brownish. Once you confirm there is fluid return the dipstick start the truck wait a minute pull the dipstick again and check the level of the fluid using the mark on the dipstick. If your fluid level is low or there is no fluid then the Hydraulics can not function properly in the transmission. If the fluid is present but it's burnt (meaning that it's not pink like it supposed to be) then the transmission has been hot and you may have other issues besides just a fluid change requirement