2000 Astro AWD transfer case issue
 

Vehicle was binding on sharp turns. Probably coincidental but I had the front tires replaced and immediately noticed a thumping from front end. Checked transfer case and it was extremely low. Filled in and all problems were resolved. 10 miles later thumping is back and binding on turns. Checked transfer case almost dry again. Refilled it and parked van for 2 days looking for puddle on ground - no puddle. Drove it and same thing happened. It's hard to get a good luck underneath to see where the fluid is going and it only seems to lose fluid while it is driving. There is still no sign of a leak on my driveway. Any ideas?

you must have matched tires on a awd. if there is differences in the rolling circumference it binds up on turns as each tire is turning at different rates.
put the same brand and size tires on the back. I'm sure your tire shop informed you of this!

no the shop never mentioned that. I have bought tires 2 at a time in the past and this has never happened. These are the same size tires as the back but of course the ones on back have wear on them. The fraction of an inch is enough to make a difference?

It doesn't explain the transfer case losing fluid though.

trans fluid level; is it high?