Reads like something is wrong, but could just be you are not used to the power band for the 3.0l.
Have a look here: The Ford Ranger 3.0L Vulcan V-6
3.0l Vulcan is a higher RPM engine, in 1999 model year the best torque is listed as 3,700rpm
My 4.0l has highest torque at 2,400rpm
Because of this the MPG is not that great with the 3.0l, and in a pickup truck that has the aerodynamics of a brick it is even worse, for any engine.
Torque is what gets you up the hills, horse power is what keeps you going, max. HP for 3.0l is at 5,000rpms.
The 4.10 ratio helps because it keeps RPMs high.
So for best power climbing hills or driving at speed you need to keep it above 3,200rpms
Stock tires, P235/75R15, should be about 27" diameter, if you go larger that will effect top speed power, because RPMs will be lower.
Assuming 27" tires:
At 70mph in 4th gear(1:1) you should be at 3,500rpm so in the torque range
In 5th(.75:1) you would be at 2,700rpm so response would be sluggish, and maintaining speed would be harder, i.e. the head wind
At 55mph in 4th you would be at 2,800rpms
in 3rd you would be at 4,200rpms
You could check compression to see if there is a problem.
Compression on a 3.0l, engine cold, should be about 170psi at sea level to 2,000ft
All spark plugs MUST be remove for a compression test.
Write down results for each cylinder
If they are lower than expected then repeat test but add a teaspoon of oil to cylinder before testing(wet test), pressure will go up but it is the amount it goes up that will tell you if it is rings or valves that are leaking compression.
Compression is the source of torque and horse power.
How many miles on the engine?
What did the old spark plugs look like?
light brown coloring means clean burning, darker color can be rich mix or oil leaking in, either lowers power