my bad 98

Ok i have a 2001 dodge ram pickup and a few months ago the original V6 engine that came in the truck threw a rod so i found a wrecked 1999 dodge durango R/T that had a good working 5.9 liter V8 engine and i put it in my truck and the motor bolted up to the 42re transmission that i originaly had in the truck so i kept it but the durango computer had a security system on it so i couldnt use that computer, which i still have, so i bought a computer that came off a 1998 dodge ram 2500 light duty so the engine would work but for some reason the original 42re transmission doesn't want to let me use the overdrive and it thinks it is over heating all the time but after 20 to 30 miles it will work perfectly fine so i need to find out why the overdrive won't work and so far i have changed out two sensors in the transmisson and it still doesn't work so if anyone can help please do.

oh and if anyone knows how to take the security system code off the computer of a 99 dodge durango R/T computer that may help