I know it's an old thread but summer is coming, so I'll revive it. I'm an HVAC tech by trade, so I can help out. Most of these trucks leak in the rear A/C lines going to the rear evaperator. Their located under the truck, passenger side frame rails, and get killed all winter long from salt which rots aluminum. My pops 96 Burb had leaks in his, GM wants like $300 for new lines and aftermarket companys sell rubber replacment hoses for $289 on egay, so I replaced the rotted portions with copper and compression fittings, and they hold well. Ebay also has compressors and receivers for $170 new, thats a freakin steal.
http://cgi.ebay.com/ebaymotors/1996...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
You can get away with just charging it, but it won't be perfect, pulling a vacuum with a vacumm pump is the way to go, plus with the vacumm you can check leaks, if you lose vacuum, you still have a leak. However a vacumm pumps sells for $300+. So if you don't disconnect the system and allow moisture in you'll be OK to just charge it.