Is this supposed to happen???

[92470]

I was under my totally stock 87' V10 this weekend changing the oil. I reached up and grabbed the driveshaft between the transfer case and front differential. While I was laying there I started rotating it back and forth slightly. When I pulled myself out from under the truck still grabbing it, it rotated a bit further and locked. I then could no longer rotate it at all. My truck was off, in park and the shift lever in 2H. Should I have been able to do this? I know I obviously wouldn't be able to completely rotate it (not Superman here), but should it have locked up like that? I feel stupid asking this but, any comments? Thanks