To add some additional info to the other posters; literally the only refrigerant-carrying part under the dash of these trucks is the evaporator. Both the input and output fittings of the evaporator stick into the engine bay through the firewall, so there aren't even any A/C line connections outside of the engine bay (unless that's a Suburban with rear air, then you have a set of fittings in the passenger rear undercarriage outside the frame rail). Assuming your shop correctly diagnosed it as a in-cab leak, there's only one part it could be, which is the good news.
The bad news is that you're pulling the entire dash to replace the thing.
Double-check everywhere else with a UV light before you go pulling the dash, just in case. Leak prone areas follow:
Compressor up top, driver side - front shaft seal behind clutch wheel, line fittings up top. Take the port caps off and listen, those valves like to leak. Pressure switches on the back of the compressor, and on the high side line.
Condenser, behind grill and horn, between the trans cooler (if equipped) and radiator - two fittings passenger side, and the condenser itself which can leak if it takes a rock or two.
Evaporator - passenger firewall. One fitting down low. Top fitting goes directly into the accumulator/dryer. One fitting on that for the low-side compressor line, one fitting for the compressor cycle switch. Can check the evap itself via inspection cam through the evap drain - on mine it's a little rubber L-shaped piece, passenger side firewall, near the evap fittings.
