An 89 IFS doesn't have Birfields, but your not far off. I'd check the CV's, and make sure your bearings aren't going out. Could also be brakes, but doesn't really sound like it to me.
I'd pull it apart, and have new bearings ready to put in. Then lube everything up real good, and put it back together and see if it goes away. Check for any signs of wear or breakage as you go. This will also give you the opportunity to check your brakes well, and make sure they are OK.
Grab ahold of the half-axle, and try to wiggle it. Should have some small amount of movement, but not much. Another idea is to pull the axle, and take it for a drive and see if it goes away.
Good luck
Erik
