[ls1nova71]

I'm just going to add my $0.02 in here and it's maybe different from others, so take that as you may.

If you are comfortable with wiring, you should be able to fix the harness, but if not, buy a new one. It sounds like you just want confirmation that a new harness is the way to go. It's your call and your money and truck.

That said, I'll try to answer the other questions to the best of my ability.

The transmission range sensor isn't absolutely needed, but does have features that are nice to have. Engine will run and transmission will shift fine without it, but there are wires that go to the PCM in it that give it info it won't get anywhere else. If you need to do a "crank relearn" for example, without it hooked up, it won't be possible. You could also use it as a neutral safety switch and back up light switch if you feel so inclined. Note however, if you buy an aftermarket harness, it most likely won't have the wires for it.

The oil level sensor is something everyone deletes. I really doubt the DD will be able to use it as it doesn't go to the PCM, and the oil has to be real low for it to trip, something any car guy would never let happen anyway!

Rear O2's will be fine to remove even if you are using cats. They just tell the PCM if the cats are working to spec, but they don't change anything. If you use anything but the factory cats they could be different enough that they could be working but the sensors could possibly tell the PCM that they aren't working up to spec and turn the check engine light on anyway. Personally, I would leave them off. Also, if you get a new harness, these won't come as standard equipment, so you will have to request that they add them.

Oil pressure sensor isn't needed either, and is another sensor that doesn't go to the PCM, so again, the DD won't be able to get a reading from it. You will have to install the DD sender in the engine to get that reading.

A 2004 won't have displacement on demand (DOD), that's a GenIV engine thing, you have a GenIII.

The MAP sensor will be held in by the black plastic part that bolts on with 3 bolts that the Vortec cover hooks on to, so if you're running the cover, that should be all you need.

You will definitely need the jumper harness between the TAC and pedal. Junk yard is a good source for them, or if you don't have one near, hit me up, I have a lot of that stuff lying around.

I think I got them all? There's a lot going on in this thread already!!