Welcome to the forum
2004 to 2006 Rangers with 3.0l engines had a problem with recessed exhaust valve seats.
TSB for that problem is here: Ford Technical Service Bulletin (TSB) 05-26-3
The exhaust valve seats were incorrect for the heads and over time would recess into the head causing a leak in the cylinder
Yes, Ford should pay for this fix but they won't
This reads like your problem, a machine shop can fix the heads but it is often less expensive and faster to just get new heads.
Low compression is of course a sign of this, a leak down test and listening at the tail pipe for exhaust leakage would confirm it.
3.0l Vulcan should be running at about 160+psi compression in all cylinders, 9.3:1 compression ratio.
Your 120psi could just be your gauge is reading low, but the 30psi less in other cylinders still indicates a problem.
The reason it will start to fire on all cylinders as RPMs increase is because with a cylinder leak the amount of time the piston spends on compression stroke is related to how much pressure can leak out.
At lower RPMs more pressure is lost so misfires would be more likely, as RPMs increase piston is moving faster so less time to lose pressure, and less likely to misfire