OBDII Linking Error

[hbar..]

Helping a friend with his 99 XJ, has a CEL but is unable to read the codes because it keeps saying "Linking Error" when it tries to connect. The reader will power up no problem, just no communication. And the key on 3x trick doesn't seem to put codes on the odometer, but I've literally never gotten that to work on any jeep. Also, the jeep starts and runs, zero issue. Nearly every thread out there on this topic has to do with bad grounds or a faulty CPS, and seems like usually the jeep won't start; this is not our experience. We also have no fuel gauge or speedo, BUT we also just removed the transmission so it's possible something got jacked in that wiring along the way. Willing to treat that as a red herring for now.

There are 7 wires in the OBDII connector (called the Data Link Connector in the FSM):

Pin 3: CCD Bus (+), VT/BR wire. This has ~4V on it with the key on. I expected this to be between 0V - 5V, so I think it's ok.
Pin 4: Ground, BK wire. This has continuity with the chassis, so I think it's ok.
Pin 5: Ground, BK/TN wire. This also has continuity with the chassis, so I think it's ok.
Pin 6: SCI receive, LG/BK wire. This has ~3V with the key on, and has continuity all the way to pin 29 on C3 at the PCM. I think it's ok.
Pin 7: SCI transmit, PK wire. This has 0V on it, but since we aren't transmitting anything, that's probably ok. It has continuity all the way to pin 27 on C3 at the PCM. I think it's ok.
Pin 11: CCD Bus (-), WT/BK wire. This has ~4V on it with the key on. I expected this to be between 0V - 5V, so I think it's ok.
Pin 16: Fused B(+), TN/BK wire. This has +12V on it at all times, which is expected. I think it's ok.

So, I've basically tested every wire and have not been able to identify a problem. The instrument cluster appears to be fed by those CCD bus wires, which also go to the DLC...so if you have info at one place, you should have it the other. What I'm hoping for by posting this is that we have an OBDII guru here who can help with some other diagnostic tests to figure out what is going on.