I have done some searching for "4L60E no reverse", and nowhere to I find that any electrical/solenoid issues will prevent reverse. I'm still thinking that this has to be a "mechanical" problem. I am eager for BP or somebody else to confirm this, but it sure makes sense in theory. P, N, and R are all simple on-off operations, needing no sequencing or adaption to road speed vs engine speed or engine load. I see no reason to complicate the P,N,R operations with PCM control. The manual spool in the valve-body (VB) can handle all those simply and reliably.
I am also eager for the OP to get live-data. We need to find out what the PCM is seeing for "gear commanded". If the PCM is seeing R from the range switch, then I am perplexed how no codes are being set. Surely the PCM should note a problem if it thinks the trans is in gear (R) and the turbine is spinning, but the wheels are not turning; that would indicate a gross mechanical problem that the PCM should not ignore!
How about the VB, is that the untouched original, that was working when the trans was removed? Is it a rebuild, or (gasp) a JY treasure? If you opened the VB up, then there are many more possible problems. I'm thinking primarily check balls.
I urge you again to check line pressure, that can be done fairly easily from "outside" the trans, and may give valuable info. Reverse takes the highest line-pressure, so if pressure is low, then I suspect R would be the first to fail...