I'm not sure on an '89 but is the front driveshaft always turning when in gear? I'm just thinking along the lines of something in the front drive system, transfer case, driveshaft etc. Something loose in or around there?
By the sounds of it when the person said when it falls off you'll know what it is sounds like it may be coming true pretty soon. I just hope it doesn't happen at any great speed because that could be nasty!

 

I had a similar rattle and it turned out that I was missing a bolt on one of the engine mounts. Check that too.

 

i just had a similiar problem. 1990 k5 . nothing could be found wrong with the system, i tried the classic 'drive till something falls out' .. guess what. the transfer case pretty much fell out. it would drive just fine, but have a constant clanky noise. have that checked out.

it didnt fully fall out, but it came apart completely exposing the insides, surprisingly it stopped clanking and drove perfectly, but its being replaced as i write this. i knew it wouldnt last long that way.