I got the hesitation problem fixed and the truck is running smooth. but know when i get up to 45 mph and higher the rpm's are low. At 65 mph i am at about 1800 and when i hit 45 mph the rpm is around 1000 to 800, any ideas of what this could be

 

It may be a vacuum leak.

 

even when the rpm 's are fine up until 45 mph, the sound even changes when i get up to 45. i pushed the o/d button and then the rpm's shot up, if it is a vacuum leak what is the best way to find it