While driving the truck pops out of gear into neutral and sounds like a little bit of a whiny or grinding noise

What is your question?

While driving periodically truck will feel like it's being put into neutral while still in drive and I hear a little bit of a whining noise and literally the truck goes in the neutral after a few seconds of slowing down while manually putting it in neutral then I put back into drive and engages and I drive again

How long has your 1999 GMC Sierra 1500 had this problem?

Several months