The 1-2 manual band is only engaged when you put the car in 2 or 1. No other times. The forward band would probably seem to slip more in all gears if the issue was with the servo for the forward band. Steve already confirmed that the input clutches/piston feed all gears and shouldn't be the issue.

Makes sense. Do you have any pictures of what I would be looking at/for, and exactly how to get that accumulator out? I've never actually removed it myself.

Also, would such a thing cause me to get the transmission code that I was getting?

Go to tech info and look for the 4t65e shift kit. That write up explains exactly how to remove it. Here it is. https://www.gmforum.com/performance-137/making-installing-shift-kit-4t65-hd-transmission-278802/

I forget which codes you had. But installing that shift kit tends to resolve long shift codes. I do have a finished accumulator if your interested. Then it'* just a mater of dropping the pan, removing 4 bolts and swapping the accumulator.

The code was a P0742.

OK, that code is TCC system stuck on. Not sure if the tune is the problem. Sounding like a mechanical problem to me. You need to get the oil pressure tested. I'm starting to suspect a possible blown channel plate gasket. However, you may have trans fluid coming out of the vent. If this is the case I suggest metal gaskets. Doing a pressure test will let you know.

I'll drop the pan tonight and see what I can find. If that doesn't fix the problem, this car is staying here and I'm taking the Bonneville to California on Friday.

The car went into what I think is limp mode on my way to work today. Shifts were very harsh, and TCC was locked on, which made it a bit of a pain to drive.

How do I do a pressure test?

When you drop the pan inspect it closely, look for any chunks or build up on the magnet.

You will need a oil pressure tester, there'* a test point on the top of the trans. You just put the gauge there and start the engine. With a long hose you can put the gauge on the windshield and drive to watch for any changes.