Transfer case noise

most of the time a clunk from under the vehicle when shifting gears (whether going forward or reverse) is a u-joint. check them at both ends of the drive shaft. (possibly 1 in the middle, not familiar with your particular truck)

Do you have 4 wheel drive ? If you do you "could " have a problem with the transfer case. The noise you describe is indicative of transfer case issues.

Worn Transfer Case Chain
Worn Out Trans
Worn U-Joints on Drive Shaft

check the front drive shaft "U" joints as one may failed and jamming on the case housing

G
Have the drive chain of the transfer case checked for excessive wear, that is about all that would cause this to make noise. There is a special tool for checking slack of chain.

I would check for a bad wheel bearing on the rear, also check the u-joints on the drive shaft.

If indeed it is coming from the drive line area, it could be a bad universal joint. It may also be a torn boot on the front axle (just thought I'd mention this), or it could be the transfer case chain has a lot of slack, causing the chain to hit against the transfer case housing internally, this would be the worst case scenario.

more than likely the transfer case chain is loose, have the slack checked, there is a gage they use to check this

It seems like no one else hear at FixYa is responding yet to both your requests about clunking in 4wd.
I see two requests....in the first a 2000 blazer from Gloeckz
there is a rapid clunking in 4wd!
In the second it clunks hard going in then won't drive!
This is rayenc11s' 98 Jimmy 4WD clunk I'm no trans specialist and not familiar with repairing or troubleshooting serious 4wd problems but it sounds like this may be a problem in the transfer case.

Since no one seems to be helping you but me I'd suggest you go to a transmission shop and ask them if this is a common problem that may lead to a bad transfer case if not corrected!

Make sure you have the proper fluid in your transmission case, transfer case and differential case first or have a quick oil change place check it for you!

Hope this is helpful.

Good Luck!
jackiaria

the vehicle has to be put on a hoist. turn the driveshaft that goes to the rear wheels,it is driven by a transfer case that is almost in the middle of the vehicle,just rearward of the transmission.
if the noise comes from the transfer case,then that is the problem.
the drive shafts for the rear have universal joints which can develop play and give the same clunk.
if it is the transfer case,it needs to have a gear replaced.
always warm up the vehicle for enough time for the engine to come down to 1000rpm before putting in drive,the fast idle is over 2000rpm,and puts alot of stress on the drive components,and over time wear them prematurely.