I need some help on this one guys. My 89 300 with 141k miles, has this shimmy that only occurs @ exactly 45 mph. What could it be?? Have new tires and brakes and wheels have been balance.

There is wheel balance and then there is wheel balance. It is extremely common for the kids that do wheel balance to use weights on the inboard lip only which accomplishes only a static balance which keeps the wheel from hopping up and down. Adding weights ONLY on the inboard lip will almost always aggravate the dynamic balance. Improper dynamic balance causes the wheel to wobble.

If the wheel wobbles AND you have ANY loose steering linkage or steering box, the vibration will usually be at a very specific speed.

My recommendation is to look at your wheels to see if there are weights on the outboard plane of the wheel. If there are not weights on two planes, this is most likely the SOURCE of the vibration.

If you are picky about weights on the outside of your wheels, a tire kid that knows and cares about what he's doing can use tape weights just behind the spokes to give you a good dynamic balance without scratching the wheel. In fact many of the new machines will tell them where to place tape weights in order to hide them behind the spokes.

In tandem with PROPERLY balancing your wheels, check all steering linkage joints, idler arm bushings and steering box adjustment.

Good luck,