Hello and welcome to Just Answer. My name is XXXXX XXXXX X will assist you with your question.
First, the oil pressure guage working erratically most likely is being cause by a defective oil pressure switch. It is located just behind the oil filter housing and is screwed in on top of the oil cooler housing right next to the engine oil temperature sensor. Replacing the pressure switch will take care of the oil pressure guage issue.
As far as the TBC warning coming up when cranking to start, if the engine cranks a long time or the battery voltage is low when cranking this will happen. Test both batteries and verify that they are storing enough cold cranking amps to easily start the engine. If you still have the factory batteries in your truck it is time to replace them.
It does sound like you may have some injector issues, what you are feeling is like a car with a bad plug wire. What it will take to determine what cylinders are missing (may only be one or can be a few) is to use a scan tool that can perform what is called "cylinder balance". This test is similar to looking at the engine with a tune up scope like used on older gasoline engines. it will show you when a cylinder is missing and then you can perform a injector electrical test to determine that the electrical part of the system is ok, if so then replace the suspected injectors in the cylinders that are showing a miss. You may have to have someone at a shop do this if your testing tool cannot perform these tests for you. They should be able to give you some good information for a diagnosis fee of 1 to 2 hours of there time. Much less expensive than the cost of a $2000 scan tool.
If you need any more assistance, just let me know and I will further assist you.