Sounds like you don't have 1 problem, but many. I don't know of any one thing that would cause these all these.
If your ECM is good, then I would guess the warning lights and inop speedo is the result of a bad cluster.
I believe there are two separate relays for the fan, one for the lower speeds and one for high. I think... not sure on that one. Or, your motor could be bad in which case a new motor is pretty cheap and super easy to R&R.
The stalling during heavy breaking could be a couple of things. Bad fuel pump or pressure regulator. Bad oil sensor switch - is your truck low on oil? Could be your brake booster has a leak in it therefore causing a vacuum leak when you hit the whoa pedal hard.
Your back up lights not working could be a bad reverse light switch.
I know when I change the tune in my truck with my SC flashpaq, all the warning lights come on when it's messing with the ECU. Don't know
maybe your ECU is bad.