The viscous coupling in the centre diff does not transfer 100% of the drive . As best if its in good condition it can transfer up to 80% for short periods but expecting to drive it in this broken condition is expecting and asking too much of it.
Particularly driving it a block then attempting to drive it up a ramp.
The viscous coupling probably overheated .
If the centre diff was a lockable one this would have been possible, as well as with
a part time selectable 4WD with a transfer case with no centre diff.
Thanks for the replies, good to know. I don't think I overheated it, because I was less than a block from home on a 20 degree morning, and I nursed it right back home. I took one attempt to use the momentum to "bounce" it up the ramp. There were no other symptoms of a bad coupler, so I think I'm good.