That is the definition of limp mode. It’s actually in second and reverse. About 35mph is to speed.
Many things can cause this, not just transmission related things. You’re going to need to read the codes. Autozone can do this for free if you dont have a reader.
Unless your in a state that does emission tests.The P0420 is a cat code and wont affect anything other than lighting the light. I know this be because it’s been on for years in all my vans.
I found one( b 0201a ) code in a promaster list and that is what it said. But early procedures and fixes are some times wrong in the books. Some B codes for Rams include A/C and A/C could be a fan problem.Though you may be correct about the system voltage, I have found that the battery light is pretty accurate. If it’s below 13v or so when running, the light will be lit.
Then again, it’s B (body) code. I suppose it’s possible to have low voltage in the body control module, but normal voltage elsewhere. Dunno.