welcome... and your wants aren't unreasonable... but they'll take some money and time.
one of the biggest drawbacks to the 5.9 is that is VERY over fuelled. to help with this you could get an SCT tuner and have someone trim back the fuel. (you'll need an AFR gauge to properly tune)
most likely, the plenum gasket has blown. the cheapest route is to just replace the gasket. the "better" route would be to get an M-1 intake, hughes air gap, or something similar. i have a 2bbl M-1 intake on my 01 quad cab dakota. I also have shorty headers, Harland Sharpe 1.7 roller rockers and the Mopar Performance valve covers with matching air cleaner. another thing that would help is a better flowing exhaust... 2.5in is big enough for the motor, but remove the cats to free up the exhaust some.
I have the AWD transfer case and on stock setup i was seeing a best of 12mpg. after them, i'm seeing a best of 16mpg.
the headers and intake manifold will allow more air in, more air out. better combustion.
The harland sharpe 1.7's (non-adjustable) are an easy bolt on mod that opens your valves earlier and hangs them open longer. (stock ratio is 1.6) this allowing more time for exhaust to get out and more air to get into the cylinder.
the orange items are the roller rockers
here is the intake manifold
here's an overall view of the engine back together.
here's the truck (should also show some of the headers)
