4L80e no overdrive

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!

Check tranny fluid level. If not low, it sounds like it's in limp mode, which means repairs needed.

Tranny fluid is clean, doesn't smell burnt, and is at the proper level. The more I drive the truck the worse it gets. It doesn't want to shift without me lifting on the pedal and it wants to jump between 2nd and 3rd gear. This truck only has a TCM for the tranny, no DTCs. Guess I'll have to find a good transmission shop and let them have a look because this is over my head!

There may be an issue with the TPS (Throttle position sensor) which is located on the RH side of the injection pump

The ignition switch issue is a concern as well.
If the start circuit does not work, then possibly there are other issues as well.

As Robyn said, it sounds more electrical. If the TPS is wacky, the tranny won't act right. If it DOES shift and hold the gear, but just not at the right time or when it should, the mechanicals of the tranny should be OK. Also look at power loss, such as caused by a plugged fuel filter or failing/failed fuel lift pump, and verify you do, in fact, have only clean #2 in the tank.

The redesigned start circuit is a concern, as well. Ign. switch electrical issues rarely come without accomplices. I suggest moving that issue a little higher on the priority list, soon.

Thanks for the input. I fixed the ignition problem, the ignition switch box on the column worked loose. Put it in the correct position and tightened the bolts. I've changed the fuel filter, cleaned the tank strainer, and checked the fuel pressure(7psi). The tank is full of fresh #2 diesel. I did a flow test by opening the air bleeder on the filter housing and running the engine. It ran the same open or closed. I have checked the TPS voltage, at idle its .05vdc and 4.2vdc @ WOT. Is this the correct voltage, and if so could it still be a TPS problem. This engine is a non turbo with a mechanical IP and a 4l80e transmission.