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.