1999 4.0 Manual Transmission. Won't start on an incline facing up. The engine turns over fine. Will start by rolling backward and popping the clutch. Fuel pressure is good. Starts fine when facing down an incline. Tank has 3/4

 

Welcome to the forum

Wouldn't be any common issue I know of that would cause that

Have a look at the CEL(check engine light)
It should come on with Key on, that means computer has booted up
If should go OFF as soon as the engine starts to turn, via starter motor, that means the computer is getting a timing pulse from Crank Sensor, no pulse = no start

I assume the comment about 3/4 tank means fuel wasn't low, and not that it only happens with 3/4 tank

If it happens all the time I would get a can of Quick Start(ether), then park facing uphill, try to start engine, if no start, open the hood and spray ether into the intake, called the 50/50 test
Try to start
If it starts then Fuel delivery was/is the No Start issue
If it doesn't start then Spark was/is the No Start issue
50/50 instant results on where to look further


Computer only runs fuel pump full time when engine RPMs are above 400, its a safety thing
When you first turn on the key the computer will run fuel pump for 2 seconds, and 2 seconds only, but it will do that EACH TIME you turn the key off and on

When you "bump start" an engine, RPMs exceed 400 instantly so fuel pump comes on full time, cranking speed is 200rpms, just FYI

1998 and up Rangers use a single fuel line to engine
And in the tank there is a check valve(back flow preventer) inside the fuel pump that should close when pump is off to hold some fuel and pressure in the system
Lets say that valve is not working as well as it should
When you park facing downhill, even though pressure is lost, the fuel still stays at the engine end of the system because of gravity

BUT, when you park facing uphill..............well gravity pulls all the fuel at the engine end back into the gas tank, so no fuel at the engine for start up, and the 2 second run time for the pump was not enough
So try cycling the key on and off 3 or 4 times to see if engine will start

Its a longshot but not a no shot, lol

 

Replaced Cam and Crank sensors. We'll see what happens