Your transmission is a four speed. Fourth gear IS overdrive, so you can't be shifting between fourth and overdrive. It is most likely locking and unlocking the torque converter that you are feeling.

Sometimes frequent lock/unlock is caused by a restricted fuel or air filter. When the engine isn't making the proper torque it will affect the transmission operation.

There is no separate TCM in this truck. The PCM (Powertrain Control Module) controls both the engine and transmission.