could be a few things...   Possibly just some trouble with the linkage, but I'd take it to a trusted mechanic to have them look at it.

when you put in park, does it jump out, or drop???
the lock may be broke on it too

look under the truck where the shift cable mounts to the trans. (block the wheels first) common problem is rocks behind the shifter at the trans binding the linkage.also the cable under the dash can break.

where abouts r u if you want a good mechanic take it to mr transmisson in welland and ask for Rob Phalen he'll get you your answer

Likely a linkage issue which shouldn't be difficult or expensive... Call Click & Clack on NPR they rock!