Clutch problem?

[Ranges]

2004 Dodge Ram 2500 5.9l Cummins with manual transmission. Going uphill or switching between gears the truck feels as though it doesn't have the same "get up" as it used to. The throttle picks up and runs high when going uphill with slow pick up or response almost as though it's straining itself to get uphill, which makes me think either the transmission or rear differential. Truck has run great until about yesterday when I noticed this problem. Could it perhaps be the clutch? Differential? I don't notice any slippage between gears or grinding from the differential. Any help or advice would be greatly appreciated.