Is the fluid dark or does it smell burnt?
If not, it sounds more like a throttle position sensor (APPS) problem. 1st thru 3rd are hydraulically controlled on the 47rh, OD and lockup are electrically controlled by the PCM. The PCM is controlled by the throttle position sensor formally known as the APPS.
If you want to be cheap, you could hard wire a grounded toggle switch to the front wire on the flat 3-prong plug on the driver's side of the tranny. When you want OD, flip the switch and it will shift into overdrive as long as the tranny is in 3rd gear. If you flip it in 2nd gear, nothing will happen till you reach enough speed to make the tranny shift to 3rd then it will immediately go into OD.
While you're under the truck, you might as well wire up a grounded toggle switch to the rear wire on that same 3-prong plug so you can command torque converter lock-up whenever you want it. When you flip the switch, as long as the tranny is in 2nd gear or higher, it will lock up. If you flip the switch in 1st gear, nothing will happen till it shifts to 2nd gear and then it will immediately lock the torque converter.