99 Astro clunking like it will fall apart

I need some help. I would like to try and figure out what is going on with my astro so I can estimate the cost of fixing it. When I accelerate from a stopped position it makes this horrible clunking noise like something is just going to fall apart. I have no idea how to explain it better then that. It actually kind of scares me to drive it. I notice it also kind of lugs when you shift from reverse to drive.
Any ideas?
Thanks

Check "U" joints 1st

Ok Thanks for the reply. I am going to take it to the shop tomorrow. One more question. If it is the U joints would something like that show up on the computer. It is going to cost me 90$ to have it diagnosed so I dont want to spend that money if it isnt going to tell us what the problem is.

Thanks again.

Agree on the u-joints and no a computer wont pick that up, had an 01 had the same noise and it was the transfer case... ouch, thats gonna cost ya!!

Thanks. Hopefully it will be covered by my extended warranty. Keeping my fingers crossed.

While it's in the shop, have all the front end components checked. Could be ball joints, control arm, Drag links, or Idler arm wear. I have a '93 and '99, The '93 ran into a guard rail due to front end wear and the '99 was just found to have a lot of the same problems.