I'm here for a friend. He has a 1991 Dakota with a 318, 4WD auto. When driving down the road it will make a horrible whining noise and it seems like it is in low range or something. The RPMs keep going hight but the truck doesn't go much faster. When it finally does shift to second gear it just drops and is like its in neautral. But, if I put it in neautral and shut the engine off and wait a couple seconds and start it back up and put it in gear it will go again. The sound I hear sounds like the pump on the transmission. Does anyone have any ideas on this? My thoughts are that the pump isn't supplying enough pressure for the clutches to work properly.

Thanks in advance!

If the motor is reving and the truck doesnt go anywhere, then the friction plates are probably worn out or like you said the fluid is not flowing. After you let it sit and it goes again is probably because the frictins have a chance to receive fluid and reseat in the friction pack inside the trans.
Smell the fluid to see if it smells burnt. What color is it? Deep red or brown. If brown then its burnt up.
This is also a symptom of low fluid level. Check it while the engine is running and the tranny in neutral, not park.