drivetrain clunk / slop

i have an 05 silverado Z71 4x4, L33 motor(5.3), 35k miles. when it shifts under light acceleration there is a loud clunk in the drivetrain somewhere. i cant seem to track it down exactly, i do notice its gone while in 4WD, so im thinking the rear diff possibly. the slip yoke isnt sloppy at all. when i shift from drive to reverse you feel the trans engage then a second later it has a thump. are their any known issues or should i just pull the diff cover and see what i find? my 07 3/4 ton doesnt do anything like that, maybe its just the normal transfer case slop. U joints are all tight. it feels like something is loose but only moves when theres enough pressure on it. Say taking off from a stop very easy it will go fine then if i lay on the throttle a hair more i feel the clunk.
almost afraid to take off the cover seeing as how the right rear drum is horribly out of round also, makes me think someone did a good long one wheel wonder and burnt up the pin and spiders. i got this truck used for a really good price so i figure ill fix some small issues and drive it till its dead.
Also the voltage seems to be all over the place and kinda random and spikes when revving off idle as if there were a bad ground. other than those 2 issues this thing is a beatifull well taken care of truck.
 
thought someone might know of any common problems in these areas, if not ill just tear into it till i find it.
 


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My 06 half ton does the same thing. Taking it in friday for the second time. They supposedly fixed it 2 months ago for the same problem.
 
Mine also does this sometimes. When you put it in reverse you feel it go into gear and if you step on the gas fairly hard quickly sometimes the tran slips and then slams into lockup. Only does the latter in reverse. They are going to look at that on friday as well.
 
The clunking that we have they supposedly told me it was the slip yoke needing greasing.
 
 

that clunking noise is normal, its caused by the aluminum drive shaft, there has to be a little play in it... now if its doing it real hard and shakes the truck then there might be a problem, if its just doing it when you shif into gears then its normal.. my 06 and my buddies 07 does it too.. its the same way with the dodge 1500's

my 01 made a similar sound and believe it or not it was the lowest leaf spring hitting on the one above it- greased it and no more noise

yea its just a light clunk, not anything hard but it just seems a little sloppy unlike my 3/4 ton.