Re: I dont have a clue whats wrong..
i just installed a new trans and when I took it out for the first drive, the truck had the same issues, ABS
light on and the Overdrive light was flashing. At what I considered normal pedal position for typical acceleration, I was only at roughly 50% the speed I would usually be at. I jumped under the truck, unplugged the switch connector and then plugged it in again and all lights were off and it shifted and acted perfectly normal. Not saying this is the case for you, but that was my experience.
1997 F250 XLT CC/SB 7.3 - TS 6-position with Swamps tunes, John Wood Towmaster Trans, Tru-Cool, D60 with Sky RSK, Hypermax 5" exhaust, 3" DP, gauges, 18" Procomp 1069's, 325/65/18 Hankook Dynapro ATM, afe stage II