I'm not sure about PA but in TX you can send in a request form for the registered title holders information. It costs a couple of bucks and you have to state your reason on the form.
It looks like the proper form for your state is
http://www.dot10.state.pa.us/pdotforms/dl_forms/dl-135.pdf that I found under Forms on the PA DMV website.
After a few weeks..maybe sooner, you will receive a letter stating the last known registrant of the truck.