After reading more and thinking about it, I'm wondering if a return line had become blocked causing the high pressure. If the line was completely blocked, it should show signs of hard starting and high pressure immediately after starting. If the blockage isn't solid, the regulator might increase pressure during high engine loads and cause the blockage to solidify.

Since the FPR is vacuum controlled, pulling the vac line off should raise pressure to about 45 psi. Im guessing this pressure is developed by the high pressure pump and regulator maxing out with no vac at 45 psi. If a line is blocked, the high pressure pump will continually build pressure until its reaching the 80-120 psi I'm seeing at the rail.

Since it starts and idles fine and only starts to choke out after load, I think I should be able to pull the vac line off of the regulator and put it back on to mimic a high load followed by idling.

Only other option I can think of is the malfunctioning regulator. Is it possible for the regulator to stick in a position and restrict flow like a blocked line would?

 

After work I was able to get a little more time to try and diagnose the problem. Inspecting the return line, I didn't see any evidence of kinks. I still need to remove it and try to blow it out with compressed air or run a line through it. Started the truck with no problems, let it idle to warm up and then pulled KOEO codes. Flashed code 11 i believe, blink-pause-blink followed by a long pause, then one blink, then pause, then blink-pause-blink. I'm reading this as system pass, so I don't think any sensors are at fault. Drove it up and down the driveway but couldn't get it to start stumbling, the idle would dip but recover when I let off the gas. Pulled the vac line off of the regulator and pressure increased from 40 psi to 50 psi immediately, then returned to a solid 40 psi after replacing the line. I sprayed carb cleaner around but couldn't find any vacuum line leaks. The only possibility looked like where I tee'd into the cruise control vac line to add a vacuum gauge. I'm going to try and get a brass hose barb and take the vacuum gauge out to see if that's causing a vacuum leak.

Edit: Inspecting the return line, I didn't see any evidence of kinks. I still need to remove it and try to blow it out with compressed air or run a line through it.
When I removed the vacuum line from the regulator, there was no fuel in the vacuum line (with truck running or not).
Also, when I shut the truck off, I watched the fuel pressure gauge drop slowly, there was no immediate loss of pressure at the fuel rail.

 

Following up:

Replaced the FPR and the fuel pressure is back to 37 psi and holding after the engine stops. No more rough idle or trying to stall at idle.

 

Check your IAC (idle air control) valve. That controls the air flow into the engine while the vehicle is at idel. If this valve/sensor is messing up it will cause you to run rich and hav every problem your having