I just wanted to update the thread with some more information I've found.
Inside the transmission is an EPC solenoid. From what I've read, it controls the line pressure for the whole transmission. The PCM controls the amperage for this solenoid, as the amperage gets lower, the line pressure gets higher. As the amperage goes up, the line pressure gets lower. Sometimes the PCM can fail and set the EPC amperage much too low, causing the line pressure to go much higher than it should.
I watched a video of a guy who had a 1994 G30 with the TBI 350 and 4L60E, just like my truck has. He was diagnosing a harsh 1-2 shift. He hooked up a multimeter to the EPC control wire and a pressure gauge for the transmission fluid. Wouldn't you know it, the amperage from the PCM was closer to 0.3a rather than the 1.07a that it's intended to be at. This of course set the line pressure to well over 100 psi at idle. He swapped the PCM for a new one, changed the PROM chip, and now amperage was at the intended 1.07a, which made the line pressure go down to normal.
I figure it's a long shot, but I'll stick a multimeter to the wires tonight. Regardless of what I find, I'll probably just swap the PCM with a new one just to see if it changes anything. I don't want to tear into the transmission again just yet, so I'm trying everything I can think of on the engine and electrical side of things.
Let me know if anyone has any ideas, I'm determined to fix this now!