First thing is to get safety glasses,block wheels securely !!...set emergency brake..,get a good flashlight.Disconnect negative battery cable...Some can wear inside the steering column,some can be a worn linkage cable ...The lever on the transmission located under vehicle Trans at lower part of trans,driver side can become loose or possibly damaged...First get under vehicle with safety glasses and observe operation of the lever where shift cable is mounted while assistant puts it in park...make sure the trans shift lever mounting nut is tight before shifting..If assistant can't get it in park use your hand and push forward on the linkage arm and it should go into park manually...If this is the problem you need to change shift cable...Some wear inside the column where steering wheel is mounted..If cab shifter feels very loose ..You will have to remove steering wheel and column cover to inspect."..The cable runs from trans to the steering column..Look between gas and brake pedal and verify cable coming up through floor board and runs to steering column...Double check shifter lever at trans is tight because it is mounted on a splined stud and it can strip the stud threads if it becomes loose..If loose remove lever and inspect splines for damage..Call dealer and make sure they have the part..Remove old cable from trans and put it in park with your hand...Remove and replace cable..Be sure lever inside cab and lever on the trans are both fully engaged into park while cable is removed before installing new cable....Attach new cable.....