actually a forklift is more of a heavy equipment or farm equipment status and does not fall under the same highway laws as motor vehicles (trucks or cars).

i dont think lifttrucks apply here

check out their website, PennDOT does more than just maintain roads
they deal with all aspects of highway traffic and safety and maintenance. NCDOT is the same way here in North Carolina.
check thier website, they do have documentation on vehicle registrations and all sorts of other info.

not saying the highway patrol dept wouldn't be a good place to start though because i'm sure they could set you in the right direction.

as i mentioned before, the company i do work for did not have to tag thier lifts, all that was required is they they have all the necessary equipment on them for highway use, now if they customer had to get permits to drive on the road? i dont know for sure, that much you would have to ask about from the local DOT or highway patrol dept.