these trucks have a fail safe that blocks out O/D until the car is 170 degrees operating temp. If the O/D blinks there is an electrical fault in the transmission usually the solenoids if you have an a340f there are 3 and ana A340H there are 4. but the whole thing is if you tps is not working correctly the truck will barely shift either. and the same with the coolant temp sensor since that acts like a switch for the o/d. i want you to read codes again. and count the blinks on the o/d light if it pulses there is no codes but i think you will find a code 62 63 or 64. i recentyl went thru the same battle but from my solenoids being bad they mechanicly messed up my trans now i have a 5 speed!!

in the mean time check this out
link on how everything works http://autoshop101.com/forms/h16.pdf
diagnosis and repair
http://autoshop101.com/forms/h17.pdf