A P2196 means the O2S11 sensor (pass side, first in stream) is stuck rich, and even when EEC compensated, it could not get the o2 to switch or go lean.
These trucks had all sorts of problems with fuel injectors sticking open or closed, which was covered under a recall years ago (which may still be applicable to your truck, depending on milage/age; you'd have to call a dealership for them to enter the VIN and check it). This (in your case, if it is) would be a stuck open injector, which would cause a hard start (sometimes slow to turn over from hydrolock) and black smoke out the exhaust, as well as fouled plugs. And if it goes on for a while, a worn out (or melted) catalyst and posssible engine damage (I've put a few engines in for this condition, which did start as a defective fuel injector).
Start with someone doing a full fuel system check, and for presssures and the ability for the system to hold pressure with no leakdown. If there is leakdown, that could be a sign that something is bleeding down.
Possibly if you could watch the power balance, then you could identify which clyinder(s) are affected by watching the power output. It should be relatively smooth without any deep peaks.
If this is running that rich, then make sure to change the oil when repaired to get the contaminated oil out of the engine.