Thanks for the welcome, folks.
We drained the transmission and put in a filter kit, but it did nothing, so we pulled the whole thing. We aren't transmission experts, though, so we're still not sure what the problem is. There wasn't any metal in the case, and when we turned the input shaft, the tail shaft on the transfer case turned as well.
At this point, I guess we are leaning towards the torque converter.
Is there any way to test a converter while it is out of the truck?
Thanks again for the courtesy and prompt responses.