I'm a high school student building up a '73 K5 Blazer.  My dad bought it back around '85, so I have no idea how it was tortured before he got ahold of it.  Its the 350ci/TH350/NP203 with D44 front and 12 bolt rear.

Here's the deal: under moderate to hard acceleration and crawling at high idle, there is some horrible clunking noise from the drivetrain that shakes the entire truck. Does it have something to do with the full-time 4wd, or is there too much play somewhere causing some slippage?  Any help or ideas are appreciated, thanks.  

had a truck do that had to replace the torque converter  

I have never had a torque converter fail so I can't confirm that one.

With the wheels blocked, put the truck in neutral and crawl under it and try to wiggle and turn the drive shafts checking for worn out ujoints or transfer case output shaft bearings sloppy.  Also check the ujoints on the front axle.  Any of those will cause clunking and vibration.  The first item I would look at is the front drive shaft.  

chain slippage in the transfer case will do the same thing also  

It is the transfer case the way i solved mine was to do the 205 conversion on your 203  

Hey, Stevoonfire. I would have to go with GM1tonOfFun's suggestion. Put the truck in neutral and see if the driveshafts wiggle. Chances are you've got some bad ujoints. Thats what my jeep had, same symptoms.  

All valid points above.

My guess without seeing it would lean towards a TCase issue.  

Welcome to the four wheeling world. The other replies are on the money for transfer case. If your truck is full time 4WD, pull up to the base of a steep hill. One that you obviously are not going to climb. Give it gas slowly but increase until you hear the noise. If it is a "BAM-BAM-BAM" and shakes the whole truck, you need a new chain. The good news is, a noob can fix it and it is relatively inexpensive. I would buy a part time kit and lock-outs at the same time. Your looking at a couple hundred bucks and an afternoon with your old man under the beast, ya can't beat it! At least you have opened up the transfer case, checked the bearings and changed the oil while fixing the chain. It's good to keep your nose in all the parts of your truck. Make sure your axle vent tubes are intact and high enough that you don't suck in any water and mud into your axle while mudding. Plan on cleaning and packing your wheel bearings at least once a year for light four wheeling. When replacing ANY u-joint, go with a spicer hardened joint. I had 40" mudders on a 1/2 ton Ford, and the cheap axle shaft joints couldn't take it. Good luck.  

i have a 203 in my 78..... think i have a t-case problem....... When in drive it a get a ticking noise (like a tap) and it would increase as a built up speed. this went on for a few months..... A few weeks ago i went to drive the truck and it was doing the same tapping then bam a loud clunking started. so i stopped the truck and threw it in H-loc. ticking noise got a lot quieter but was still there....... i have new drive shafts (custom built) front in back (2 months old). Tranny was built 1 year ago.... can someone help on the comfirm if it can be the chain belt?  

I sound like you need a new drive chain. WHen you put it in loc the tension of th chain was changed  because even though axles have same gears there are differences in rolling radius on them and loc puts different torque load on chain.  

I agree with snoman, as the chain gets stretched the noise/slippage will get progressively worse, and the loads are different in H-Loc.  

I had my t-case rebuilt with a new chain and installed a part-time kit at the same time, the noise and slippage went away, plus i'm getting much better gas mileage since it's running in 2WD most of the time.  

The problem with most of those kits is that you give up full time option completely and full time is a asset in poor conditions. There used to be a kit out there a while ago that gave you the option of part time and full time as well as conventional "locked" 4x4 drive. I have a old J20 Jeep with full time quadratrac that I have owned for 23 years now and for what I use it for I would not give up fulltime ability as I know what it does for you.  

thank u snoman............ i even went to the point where i took the front shaft out and it still does it loud in high but not so bad in LOC.... (yes i do have the conversion kit)...... thanks again  

I tear down is warranted here. It may be a problem with the kit itself because some were poorly designed and could develop issues with needle bearings internally in diff in conversion. I would fully inspect and and replace chain while apart and do use 30 weight NON detergent oil in it not 10w40 which many recommend because they do not know any better. Case was spec'ed for 30 weight non detergent when built. You can use non detergent 20 in winter.  

ive been running i believe its 80w-90 race gear oil by valvoline. (dont quote me on the exact # on the wait of oil).......   Im picking up another 77 k-10 for $600 in good running condition and going to swap out transfers.... if that solves the tapping noise i will take my old 203 and have a pro rebuild it to hold up to my 454.... im pushing right at 520-530hp.... where are u located snoman?