it only happens when it get colder when it wont shift what part need to be change out
Automatic or manual transmission?
If manual, can you shift through the gears when engine is off?
If automatic, you can try a fluid change, but on a twenty year old vehicle with unknown maintenance history, unknown mileage, unknown drive system, and unknown engine it might be best to take it to an independent mechanic.
Way too little information provided to make any guess.
When was the last time the transmission fluid was serviced?
The big steel piece between the bumpers.
I was thinking everything above the area that touches the ground.
I was thinking when someone says, it’s broke, it don’t work, what’s wrong; it just might be the loose nut on the steering wheel…
try letting the truck warm up for 5 minutes before driving to see if it stops. my daughter leased a 2018 hyundia elantra. and from the beginning when it got below 32 degrees the same thing would happen. let it warn up for 5 minutes and it would not do it. it was a lease and she did not want to waste the time to bring it back to the dealer so she would just warm it up a little. but in your case you can live with it till the tranny dies or bring it to a tranny shop.