It could be any number of reasons. Faulty compressor clutch, faulty compressor, blocked condensor, blocked evaporator, faulty cabling/ air direction flaps in the cab, blocked TX valve, overcharged system, undercharged system, faulty viscous fan, stuck heater tap, etc.
Check that compressor clutch is engaging as a first check..
Resurrecting this thread as my old man has a 96 as well and has the same problem.
He recently had a new compressor installed, new valve (tx or expansion?) and had the system evacuated and recharged properly. Even the a/c guy said it improved,but he wasn't happy with how cold it was but didn't know what else to do. :facepalm:
Since I'm no expert I wanted to see if there's something specific that the 96's might be known for in regards to a/c not cooling well?