Overdrive Light Flashing?

Hi there, I just recently did a tranny swap in my 96 bronco after destroying my old E4OD. I put the new tranny I got on craigslist in and it ran flawlessly for the first 20 or so miles and then the problems hit me like a damn wall. The first problem I noticed was that my tranny wasnt shifting into overdrive, then the overdrive light starts flashing. This is where I start questioning things...WHY IS IT DOING THAT??? I put 3 gallons of tranny fluid with the engine running like im supposed to, but im just not sure why it isnt shifting into OD

another one for yall, shortly after this OD problem, everytime I start up my truck, the RPMs are higher than usual. Since its the base model, I dont have a Tac so idk what the RPMs are running.

Any help is appreciated! Expect me to come back soon because I have more problems with it that im still trying to figure out.

I also forgot to mention that its getting me 4.5mpg....thats not acceptable. Having an emissions problem isn't helping either.