I did a lot of A/C work years back. Because units are so well sealed, even with a minor leak, they're usually fine inside. Plus the charging medium contains oil, so the whole guts of the thing had oil blown thru it constantly for life & that oil is still in there coating things.

The electro-magnetic clutch/drive unit on front sometimes funks up, an easy thing to swap. 9 times out of 10 they just lose their reefer charge. Hook up a battery to truck & switch on A/C with ignition in run position & you should hear a loud click as clutch drive kicks in. If so you're more than likely good to go. No click & I'd trace wiring & fuse before giving up on it.

You could swap the unit, evacuate with a vacuum pump available at some rental yards, recharge with the new refrigerant & you're fixed. By pulling a vacuum it removes the old & makes room for new refrig & unit pretty much only takes as much as it can use. I believe you charge with unit running on those to top it up. Web search?

Most shops have a flat rate for changing to the new 'Approved' stuff so if a shop does it you know ahead of time how much it will cost, certainly the easiest route & pretty cheap. Shop around because some will try to spring a hidden charge for 'Hazardous Wasted Disposal Fee for old freon', or add on exorbitant price per lbs of refrigerant charge, or both! "Oh, didn't we mention that...?"

Also you might do a net search because there's some choices in which refrigerant to use. Some sharp A/C guys have posted up alternatives that supposedly cool much better than 134.

Yeah, Shovelhead, Grave Digger HAH! That's calling a spade a spade for sure...