99 transfer case question

1999 chevy z71

This is a stupid question but how do I check the transfew case and front differiental fluids? When I get at speeds above 40 mph (in 2wd) the front drive shaft or something starts vibrating badly. I have completely removed the front drive shaft and the vibration goes away. I have completely replaced the front drive shaft, ujoints, and yolk but the problem is still there.

Thanks

Transfer case fluid change (includes info on level check plug):
http://gm-trucks.com/home/content/view/148/0/

Front diff. fluid change (includes info on level check plug):
http://gm-trucks.com/home/content/view/144/0/