4L80e no overdrive

[rfree82]

Hi, i'm new to the site and diesels. I just purchased a Chevy P30 step van with a 6.5L non turbo and a 4L80e trans. The truck seems to run great but it will not run over 50 mph and it will not shift into overdrive. If I go down a decline and manage to reach about 55 mph the truck will shift to overdrive and remain there until I slow down and it down****s. The truck shifts through the rest of the gears OK. A few things about the truck... It was originally a navy owned truck used on a base. The ignition switch works but the start circut does not, someone installed a push button to engage the starter. After further inspection I found the idle stop solenoid was laying in the valley. I put it back in place and not it will not shift without lifting the pedal, that may just be coincidence. I have changed the fuel filter. I'm lost so any and all help will be appreciated!