My 2001 mustang wouldn't shift out of park, overheated...?

Check the transmission fluid levels first; that will often cause it to stick like that. If it isn't that, it's also often an issue with some sort of transmission sensor that talks to the ECU. You may want to check the brake pedal as well, especially if it's an automatic transmission since a bad sensor or low pressure can cause it to not engage.

Get it scanned at a shop with a high-level scan tool (although high end ones can be had for as little as $69, see here: FOXWELL NT301 OBD2 Scanner; ABS testing ones can be had for $100: INNOVA-CarScan-Pro-5210). Get the codes and see what the parts cost to install, and what needs to be done. IMO if you're going to work on your own cars, buy a nice scanner for $60-100 and keep it in your toolbox.