Remember to check fluid level in neutral. Park will give you a false high reading.
Assuming you are talking about a 98..... On the drivers side of the trans, about halfway back, is an 8 pin connector. Unplug it. Take the truck for a ride. Bear in mind, you will have ZERO automatic function. If you don't tell it specifically to shift, it won't, and it will start off in whatever gear you happen to put it in. So, start out in first, and shift to second/third at appropriate times. You will not get TCC lockup, or O/D in this test, so, don't go far.
If it all shifts nicely, when you tell it to, you have an electronic problem. If you still have issues, you can try adjusting the bands..... if that doesn't help.... rebuild time.