If I take the valve cover off the engine am I going to find that the rocker arms are all under tension from the valve spring? I'm wanting to remove the rocker arm to cylinder 5 and 6 to run a test to see if my exhaust valve is leaking

 

You won't be able to tell if exhaust valves are leaking from valve cover area, but yes you can remove a valve cover and loosen the rocker to be sure valve is closed

Generally to test if a valve is leaking you put the piston in that cylinder at TDC and then inject air into the cylinder with an air compressor, then listen at the tailpipe, or throttle plate, for leaking air

There is also the Dollar Bill test, an oldie but goody, lol
Google: Burnt valve dollar bill test

Won't tell you which exhaust valve is burnt, but doesn't matter, because if there is one then head(s) need to come off regardless
Do this test FIRST on a vehicle with no mis-fire issues, just so you know what to look for

The valve and valve spring are separate from rockers, so rockers can be removed, the rockers are held tight by the small springs in the Lifters when engine is off

If you rotate the engine manually and you can see when the cam pushes up on the Lifter, which pushes up on the push rod, which pushes up on the rocker, and rocker pushes down on the valve, opening it
All the parts are separate, but the rockers do need to be adjusted properly to work

 

I have a misfire code on cylinder 5.
When I open the valve cover are any of the rocker arms going to be contacting the valve spring under tension? Or is there going to be play between all of the the rocker arms and the valve spring? My concern is not being able to loosen the rocker arm bolts. Or do I have to turn the engine by hand until the tension is off of the valves for 5 and 6?

 

You turn the engine by hand until the valve you want to check is fully closed, stem is up all the way
You can rotate the engine until you see the rocker push down on the valve stem and valve opens, stop rotating when valve has been pushed down all the way, you can tell
THEN rotate the crank by hand 1 FULL turn, 360deg, then that valves rocker will be fully up so valve is fully closed

The rockers always have tension from the small spring in the lifter even when valve they operate is fully closed

 

From a PM

It could be 2 leaking exhaust valves OR head gasket between 5 and 6 is blown

But yes, its a head or head gasket issue
Pull both heads and have them serviced or replace them
#2 and #4 are also low, so exhaust valves would be my guess on all 4

 

FROM PM
1991 to 2008 3.0l from Ranger or B3000 will work, doesn't have to be flex fuel, but you do need to swap intakes
2002 to 2004 Should match directly, should is the key word, an autowrecker may have that info, I don't

You can remove spark plugs and valve covers then rotate engine by hand and measure the travel of each rocker, if one doesn't open as far as the others then its cam lobe is bad, but very unusual occurrence in a 3.0l