Sorry to say there is no adjustment as that is a hydraulic clutch. Your description of slippage under load and needing to let it "build power in a lower gear" both say it needs a new clutch.
They are not a lot of fun but not really difficult. RockAuto shows kits from $80.00 to $175.00.
The hardest part is getting the transmission and transfer case out if you do not have a lift or transmission jack as they are heavy. You can remove them as individual units if that is the case it just takes more time. When you have them out it is a good time to clean the tunnel and check all the U-Joints as well.
The exact parts you need will depend on which trans and flywheel you have as GM used a couple different ones on that truck.
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Monday, May 28th, 2018 AT 8:50 PM