Hello and welcome to just answer Dodge. The 5.9L has had issues with the valve seats and will cause misfire(s). I have seen this at mileages as low as 40,000 miles. If you are at 150,000 miles, that is really good. There have been valve seat issues, and also combustion chamber cracks found in the cylinder head(s). One other thing I see from time to time on these engines is the intake manifold "belly pan" that can leak and cause misfires and other driveability problems. One way to check this is to look down into the throttle body with the throttle wide open. If you see a lot of oil sitting on the bottom of the intake pan, then the manifold pan gasket is leaking. However, it probably is a valve seat/cracked combustion chamber that is causing your #5 misfire because the misfire is only on that cylinder. A compression test may not always be a good indicator of a valve seat issue. A leak down test would be a better way to confirm this.
Hope this information is helpful, and let me know if you need more assistance with this.
I will look down the intake.
1. Is there any way to see that valve seat without pulling the heads? Would one expect to see a burned slot between the valve and seat?
2. Should I do the leak down before pulling the heads to be sure, or are you confidant there is a valve problem from your experience.
3. The thing that bothers me is that I remember the instant or trip it started misfiring. When stopping at a sign or light it really makes this misfires stand out. Between this moment and another it seemed to have started. Can a computer reset cause such a thing n any way?
4. Do I have reason to suspect of the short block or lower end as well?
There really is no way to visually see if there is a valve seat problem without pulling the heads, and even if you could, you still would need to pull the intake and exhaust manifolds to look down into the intake and exhaust ports. Yes you would need to do the leakdown test before pulling the heads. A computer "reset" would have nothing to do with this. It is purely a mechanical issue. As far as a short block replacement, that depends on how much longer you plan on keeping this truck, and also if you have a reason to do that (oil consumption, lower end noises etc.). One other thing that I was thinking is this; Have you noticed unexplained coolant loss? Do you have to add coolant to the reservoir bottle every now and then, but do not see any leaks? If so, this is another indication of a combustion chamber crack.