From park, you can get 1st by shifting to it directly. If I'm towing I turn off OD straight away. Once I'm rolling I advance through the gears manually at 2200-2000 RPMs, then hit OD. When slowing or stopping, it's possible to shift down to 2nd manually, but not 1st. The trick to getting 1st when decelerating or stopping, is once you're at the RPMs desired move the selector down to 1st, then quickly shift to 2nd and back to 1st. This works to get 1st every time. That locks the transmission in 1st until you move the selector. I figured this out by accident. When I towed the trailer to VA, I was having to come to a complete stop, shift in to park, then to 1st to get that gear to get rolling. It was a pain.
I'll probably have to bite the bullet and send the PSOM off. I'm trying to exclude all other factors first. I don't think the PSOM from my '90 or '94 are the same. Maybe the one in the '90 will work??