2001 Dodge Truck no overdrive

Last week a few times my truck was heisitant to down shift to pass another car, I had to let off the gas, then back into the gas to get it to shift.

2 days ago while hauling a light trailer, I was going down the highway at about 65 and put a little more gas to it and a transmission light(possibly a check engin light) would come on, and if I let back off the gas the light would shut off.

as of yesterday, it seems the truck will no longer shift into overdrive, witch leaves me baffled cause I was thinking of an electrical issue with passing gear, or vacum issue.

please help! Thanks for your time.