My '89 745 GL w/ auto trans refuses to shift into 4th gear. I tried switching O/D relays, but that didn't change anything. What next? Is it the transmission, or could it be the O/D solenoid? I have also noticed along with this, that it shifts very quickly from a stop into the next gear. The idiot light comes on when I push the O/D button, but nothing happens. Thanks in advance!

Is very likely the solenoid. See if it is engaging as per discussion here and at http://www.brickboard.com/FAQ/700-900/. Turn on the key to acces., kill the doorbell while you open door with head out- you will hear an engaging solenoid when toggling the override. My tranny man says when 4th gear really goes you shift into no gear at all. check solenoid wiring too.

I would deff check the OD sole first thing, if its kicking into gear to early check your kickdown cable, though the cable should not stop it from going into 4th gear above 60-65 mph anythings possible, though check the sole out first!