I have a 2006 silverado 1500 4x4 and i put a true track in it today and put enough shims in it so it was tight and everything went together just fine filled it up with some valvoline 80w90 conventional oil. I took it for a test drive and on deceleration it was clucking at a constant pace, give it a lil bit of gas and it stops, as long it has a little bit of torque on the driveshaft it doesnt clunk. But as soon as you let go of the throttle it clunks. Also when i change from reverse to drive and from drive to reverse it clunks pretty loudly. Any help appreciated.

Get under it and twist the driveshaft with your hands. Find where the clunk is at, or at least eliminate the u-joint itself.

Because this isn't a stock application, and we can't see inside your diff, we can already assume something isn't correct.

i made a video to show you guys what its doing. Also when its in neutral and in the air when you turn a tire there is a lot of slack till the driveshaft moves. http://youtu.be/dLiTk8MFLvo

That clunk sounds like it is coming from the rear diff, but I am not there.
Can you tell where the sound is coming from?
When it is clunking, put your hand on the diff cover, and see if you can feel it.
Next is to take the rear diff cover off and inspect & look at gears etc inside.
Something is shot.

I seem to remember a previous thread that you had started about this problem when you had asked opinions about the True Trac, it'* possible that your original problem was not with the OE locker but with the ring and pinion gears, I'm far from a diff expert but if it was me I would take it to a shop that specializes in differentials where they'll check the ring and pinion gears for wear, proper mesh, lash, etc. I'm leaning towards the ring and pinion as you had stated earlier that while under torque there was no noise, just while backing off. Sounds like gear slop to me.

2004 Bonne SLE
1971 Chevelle SS 12 bolt with 210k original miles

When I was in Chevy parts, 2000-2008 Silverados were always in the shop having R&P'* replaced. These trucks were mostly "family cars" that never saw an inch of off-road or pulled a pound of trailer. Just poor design, evidently. Between the R&P'* and bad tires (stock 16"ers), they wore us out.

Though I had a set of gears on the shelf with my name on them, my 2000 was one of the good ones that never gave me ounce of trouble.