Gear shifter moves but wont go into drive or reverse only to park. car starts fine.

the shifter wont move, or moves to park but park does not work?
try to be clear.
are you on flat ground?
are you against a curb
did you wiggle the steering and turn the key and wiggle the shifter to park? at once.

there is thing called park lock out but works moving from P/N to gears active, R/D/L. usually going to N/P is no problem. unless cables are rusty.

check the shifter mechanism for binding up and not moving freely into positions. Not starting in park but in neutral indicates something is out of alignment with the shifter. May just need cleaning and a bit of oil/grease to fix.

You have to locate where the park ignition switch is located. Some are on the transmission and some are in the column under the dash. It probably got moved.

you can shift it at transmission by hand to neutral or park and have someone inside start the car and you manually put in gear under hood and drive it to shop or garage, wherever

Do you have your foot on the brake when trying?

It sounds like the linkage at the transmission has come loose. Have someone get inside and move the shift lever from gear to gear so you can see where the linkage is on your transmission. You'll have to climb under ypou truck to see this, on the drivers side. check to see that all the bolts are tight. Your switch that allows you truck to start in Park or Neutral is located there.

shifter or linkage is damaged or out of adjustment or tranny park gear is damaged!

you need to have the shifting cable lubricated or replaced.

set your parking brake, insert your key into the shift interlock, located next to your shifter, push all the way in, move your gear shifter to neutral, remove key, start car, release parking brake, drive away, have it serviced ASAP

Sounds like the car is just slipping out of gear. Keep your hand on the shifter when you back up next time. See if you can feel the shifter move when the car comes out of gear. If the shifter moves up when it happens keep pressure on the shifter holding it back like you dont want it to move forward. And of course if the shifter moves back when it comes out of gear, put forward pressure on the shifter so it doesnt slip out. This might work for now but but i know i wouldnt want to spen 700 for that ****. Let me know how it goes.