2011 Ford Ranger Pricing and Specs

By connecting the Ranger to a diagnostic computer, you should be able to work out what's constricting the engine power. Modern engines constantly monitor everything from boost pressure to fuel pressure, and transmission temperature to alternator output. Somewhere in those hundreds of parameters will be the problem and it should be fairly obvious to somebody who knows how to interpret the codes the car's computer will spit out.

Meantime, it could be that the engine's installation skipped hooking up a sensor or piece of wiring and that has caused the engine to go into limp-home mode which limits the power the car will produce in the interests of self preservation. On a really simple level, have you checked that all the turbocharger's plumbing is intact and the joints are tight post engine installation? Loose plumbing joints and cracked or split plastic tubing are a common way to lose turbo-boost pressure.

Normally this would be an issue with the glow plugs, but the 2.5 turbo-diesel Rangers seem to occasionally have an issue with the assembly around the fuel filter (which includes the water trap and primer pump) sucking in air. This can be fixed by replacing the unit as a whole. Similarly Ranger owners have reported trouble cold-starting their 2.5 TD when the valve stopping fuel flowing back into the tank from the pump fails. Try priming your fuel system a couple of times before starting and if it starts up fine then replace that valve near the fuel pump. 

How did you determine the gearbox oil was low? An additive won’t do anything for you, except maybe fix a leaking seal, but you don’t say if there is a leak present. Take it to a mechanic.