Engine Rev Issue - 5.7l

[Duzey]

I noticed my Suburban has a slight idle bounce when sitting in park running. But what got me is when I am driving it, and say I let off the gas going down a hill, my engine revs up and down over a 600rpm variance. Looks like i am putting it in neutral and revving it up slightly.

 

I have seen this at idle, but never when im coasting in gear. 92 5.7L

 

Also the rumble/howl when i am slightly on the gas/letting off the gas is not the U-Joints. they seem to have zero play. Any other suggestions? Rear end was changed out to 4.10's a few years back.