Good to know that I'm not going blind. Thanks for the help.
This solved it! Adding resistance inline fixes the issue. I tried 10k to 10M and they all worked. I used the BNC cable that came with the 6485 (0.5m I think?)
Where would I find the 6485 instruction manual? I couldn't find it in the 6400 family downloads section. I did manage to find a copy hosted by Purdue University.
I am using a 6485 to measure DC photodiode currents around 90pA-100pA. When the photodiode is in photovoltaic mode (no reverse bias), the 6485 overranges when put into the 1nA and 10nA ranges, but doesn't overrange when in the 100nA range or higher. When the photodiode is put into photoconduction mode (reverse bias applied) with a modest voltage (500mV to 1V), the 6485 no longer overranges in the 10nA and 1nA ranges. The 6485 also doesn't overrange when 100pA is applied using a voltage source and 10M resistor. Why does the 6485 overrange when the photovoltaic mode? Is it perhaps due to excessively high source impedance?