I have a 92 e150 van with a V8 automatic transmission with overdrive. After sitting in my driveway for two months it won't shift into overdrive but goes into what feels like neutral at 45 mph......What's the problem?

sadly you will need to pay to get the transmission rebuilt .

run the fault codes
make sure that the transmission oil has been changed in the last 60k miles

This 4.3 V6 is just a 350 V8 with 2 cylinders missing and even a V8 trans will fit/interchange physically being the same bolt pattern and size. If it's the same shift pattern and both transmissions are overdrive transmissions, yes, it will interchange. If it's GM, it'll swap. From 1988 the automatic transmissions have a multi bolt pattern and Pontiac, Caddy, Olds and Chevy, all will bolt up.

Unless you really know what you are doing it would be well advised to take car to transmission specialist. U have a sick puppy in urgent need of medical care.

On the throttle body is a shift cable check the setting

Hello tbone.
There were only two transmissions available in the E150 in 1988.
If your van is a 3 speed it is a c6.
But if an overdrive it is an AOD 4 speed.
Hope this helps you out.

check all around the transmission for a connector not hooked up for your overdrive
check freon

You can swap just the transmission unless the shifter is on the column,then you will need the column as well,You don't need the engine if your is still good.

Under the engine, between the rear of the engine and the front of the transmission. It will be on a mount with an oil cooler, (it has metal fins) on the driver's side.

a solenoid problem if its an automatic transmission,if its a manual(with clutch)then its probably the syncros bad