31" tires lowers the rear axle gear ratio so you lose low end power, but get better MPG because of lower RPM at highway speeds.
3.0l Vulcan engine is a high RPM engine, torque range is 3,200-3,800rpm, so it needs to be above 3,000rpm to have any power.
Lack of power can be a few things, best place to start is by getting a vacuum gauge and test the engine, vacuum gauge is $20-$25.
Good read here on test here: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine
Low vacuum could mean low compression so that would be the next test
Go here to get your current rear axle ratio off your drivers door label: http://www.therangerstation.com/tech...le_codes.shtml
label will also have stock tire size, get that diameter, usually 28"
Then go here and plug in your numbers: http://www.therangerstation.com/tech...ioChange.shtml
means actual ratio now is 3.37
New ratio 4.13 in rear axle would change truck back to 3.73 performance that you were used to, 4.10 is the closest ratio for that change