Based on your description I am assuming it "feels" like the transmission slipped in to neutral and this "feeling" has nothing to do with the shiftier position. I have a 1999 with the 4r70w.
Based on that, have you checked the trans fluid level? I had a similar issue with mine after a transmission fluid and filter change. I was half a quart low and if I was on an incline I could feel the transmission slipping (feels like it is not in gear and able to rev the motor without going anywhere). After topping it up the issue went away.
Make sure you check the transmission fluid while the truck is warm (has been driven and up to operating temperature, engine temp gauge should be in the middle) and idling in park on level ground. There are two holes on the dip stick and a cross-hatched section above the 2nd hole. The fluid level should read at the 2nd hole near the cross-hatched section. If it is at the lower 1st hole this means you need half a quart to get it full. Do not overfill it.
You shouldn't be losing any transmission fluid between changes so this could also indicate you have a leak somewhere.
If there is a leak in the system some of the common places will be the transmission cooler lines (if you have the tow package) and the transmission pan bolts if a recent transmission service was done.
