Welcome to TRS
Good read here on the 2.9l:
http://www.therangerstation.com/tech_library/2_9_Page.shtml
And tips here:
http://www.therangerstation.com/tech_library/Kunz_Korner.shtml
Yes, the ticking was a common issue as the 2.9l got older, and yes there is a loss of power if you hear ticking because valve is not being opened and held open long enough.
2.9l has hydraulic lifters AND rocker adjustment, 4.0l didn't have adjusters.
Hydraulic lifters do two things, they pump oil thru the center hole of the push rod to oil the rocker and they get "stiffer" as oil pressure increases, higher RPM.
It reads like you have a collapsed lifter or a few.
Inside each lifter is a spring and valve, the valve allows oil to flow in the lower part of lifter but not out, so when the lifter is compressed the oil is compressed and flows out the hole at the top that lines up with push rod hole, the compression also stiffens lifter.
Spring expands lifter after it compresses
Broken spring and/or valve is a collapsed lifter, and it can work and then not work so adjusting can fix it but it will come back.
I think you have to pull the heads on the 2.9l to get all the lifters out.
The way to adjust 2.9l rocker with hydraulic lifter is to make sure lifter is down all the way on the cam, cam lobe opposite side, tighten until there is no play in the rocker, then turn adjuster one full turn then half a turn, this pushes lifter spring down about 1/3 of the way.