Sporadic clanking sound at high speeds (98 Blazer 2dr / 2wd)

I'm experiencing sporadic clanking or banging sounds at speeds above 45mph.  The faster I drive, the more frequent the noise occurs.  Although I'm not positive, I believe it's coming from the front end.  I don't feel any vibrations in the steering wheel and accelerating or turning makes no impact on the noise.  Also, it's not a consistent/rythmic sound such as a nail in a tire (clank, clank, clank, clank...).  Rather, it sounds very similar to a couple rocks in a dryer (clank.... clank clank......... clank... clank clank clank).  I've taken it to 2 repair shops without success.  Sometimes you hear it at 40mph but sometimes it won't appear until 65+. 
 
Any ideas?        

it could be a few things if its like mine i have had u-joints click at speeds like that or wheelbearing click kinda tick sound.  ive even had the fan shroud go into my fan when my motor would slightly move idk i hope this helps im sure somebody on here knows

agibson, thank you for the quick reply!  I took it to a 3rd shop and had them specifically look at the u-joints.  Instead of the joints, they recommended replacing: 2 motor mounts, 1 transmission mount, 2 lower ball joints, idler arm, center link, and an upper control arm bush kit for a grand total of $1,400.  I don't doubt the idler arm or ball joints because I've experience loose steering and occassional clicks while making sharp parking turns.  The motor & transmission mounts are new to me though.  If it were a mounting problem, I'd have assumped some noise while driving over bumps (nothing).  Anyone else have problems with those items?  And which problem or combination of problems is the most likely cause of the clank / rattle at 60mphs?