I recently replaced my transmission mount on my 1992 SC300 5spd. It didn't look shot when I first looked at it underneath the car, however once I got it off it was clear how bad it actually was. My ride has noticably improved and everything is much smoother. This is for a 5spd, however I am SURE that the procedure is about the same for the others as everythign bolts up the same for the transmission mount (the positioning of the bolts is different, however I am sure anybody that is willing to tackle this task can figure it out).
When you get under the car, you want to jack up the transmission a little so there is not much weight sitting on the mount. I had to use 2 bricks to hold up my jack higher because my car was already on jackstands so I could get underneath it.
Once I got the transmission lifted a little bit, there were 4 bolts on each side of the transmission mounting plate which bolt to the car (this isn't what your replacing, it goes below the mount. It also has 4 nuts you need to take off that connect directly to the mount.
After you remove this, there will be 4 bolts, and depending on which car you have, they will be positioned differently, however you can see them all, simply remove all of these from the transmission.
The mount will now drop out for you to remove.
Replace the mount and redo the steps in opposite order.
Now drive your car and see how it feels! It shoudl be a lot smoother and shifts won't be as jerky, especially on 5spd cars!
BTW: I got the mount from Sewell Lexus for $47 I believe, not bad at all.