Need help. Oil Pump may be bad. How do I change that?

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Oil light comes on during low idle after trucks been running and warmed up. I have also lost of power while I shift gears. I took it to mechanic to see whats going on, because I could not pinpoint the problem myself. The mechanic told me it might be the oil pump and it would cost me $1500. Is that about right? how difficulty would it be for me to change it myself?