I have replaced these lines several times on various GM products. In regards to just "cappin off" and not having rear AC in a Suburban, well if you had children back there you would probably qualify for arrest, really, no rear AC in a Suburban is not a good idea.
Did they show you where the leak is? many times when you just can't find the leak it gets balamed on these lines, they can be repaired, the dealer has as an "essential tool" a repair kit for these lines.
So it is a $2K job, always wondered what it cost the customer. One thing that can add too the cost (really two things) is that one of the lines that goes to the expansion valve (at the rear evaporator) likes to sieze up, this means you would need a new evaporator and expansion valve. Second, GM compresssors of this vaitage like to leak where the compressor case halfs join, easily a 50% leakage rate with these compressors.
Are you being told you must replace rear heating lines also? (they carry coolant)