The following information is intended for novices, "old" heads probably already know all this. That said :
In the schematic, the "Low" and "Hi" relays could be 4 pin relays while the "4 Hi" must be 5 pin. 5 pin relays have an 87 and 87a pin, some have pins 87 and 87a both normally open (NO) and...
Be careful using the "lift" keys. Simple geometry says if the control arm pivot points remain the same you can only raise the vehicle a set amount before contacting the droop stop, regardless of which keys are used.
I think a lot of the greasable u-joints that break are because of being installed improperly indexed. The cross should be installed with normal direction of rotation compressing the grease zerk (threaded) passage.
Most if not all with the bleeder screw is because the top of the engine is higher than the top of the radiator.
Vortec heads are known to crack, especially if they've been hot (overheated), so don't overlook that possibility.
Hopefully you have it fixed.
My ATRA books say that 4L60Es with a KCD code (5.7L, 4WD) or at least some of them came from the factory with the "Corvette" 2-4 servo. Might want to check it out before laying money out for a new 2-4 servo.
I don't know but it seems to me that the cylinders with wet plugs are the ones misfiring and all that raw fuel is being sensed by the pcm, causing it to pull fuel and driving the other cylinders lean.
The old black box pcm had two boards inside, GM issued a TSB and kit to fix a bad ground issue...
The regulator is as you say on V6 models (as shown in the Haynes manual 14.11) but your C1500 is a 5.7 correct? The regulator is on the passenger side as the "fuel meter body" is rotated 90* from the V6 orientation.
Might want to check the fuel pressure regulator in the throttle body, hard to tell if it's leaking when the engine is running though. Seen them leak enough extra fuel to foul a spark plug. If it's leaking and the injectors are good otherwise then a regulator rebuild kit is available.
Rich on bank 2 could be the fuel pressure regulator as that's the side it's on. And if it's constantly rich I would think the computer would pull fuel to lean the motor out but only in closed loop, should be running off a fuel table in open loop until it warms up.
"mine is unique under my hood", "Took 3 fuel pumps and 2 FP assembly's to get everything together", " I just put a new fuel pump in it, had to frankenpump the assembly". So this truck has had an intake system change to a throttle body injection system? What computer is controlling the injectors...