Check the throttle cable at the pedal end, there may be slack..that will cause slow acceleration if the cable is stretched. There is a do how to fix this. Driving in 4th gear at 45-50 is lugging thus engine. If I am around town or getting on the interstate I am in 3rd at that speed. I usually go into 4th around 55 to 65..4th from there, and 5th around 75..can get up to 90+ in 5th..I usually do 70 to 80 in 5th all day, and has alot more acceleration when needed. I would start with the throttle cable and perhaps change your shift points. Does this ranger have a tachometer?
Nothing on my ranger is stock, yet a stock 3.0 5spd is a good combination in the ranger, the auto behind this engine sucked in the Aerostar and ranger...in the Taurus it's great. An old friend had a 99 3.0 Taurus and I drove it often..this 5.4 expedition wanted a peice of it one night on the interstate long ago..I put in drive from overdrive, heavily accelerated..left that expedition far behind.
People rag on this engine, yet thier ranger is an auto or they don't drive it properly. People forget that the 3.0 was also put into the Aerostar in 86..not just the Taurus. From 86 to 90 there were many 3.0 5spd Aerostar vans..most were worked hard..they are still used to this day. For those who say this is only a car engine...wrong, the 3.0 has been a work horse from the day it was made.