I had a real bad vibration in the shifter of my truck that I could tell was coming from the undercarriage of it. Well it turns out I have a wheel bearing going bad but needless to say I also checked my rear transmission mount (the only mount) on my truck. The mount itself is located at the rear of the transmission right next to the collector on your exhaust system on most trucks. (See pictures) The manual states to take a large screw driver or prybar to the mount and see if it flexes away from the frame cross member significantly. If you can pry that thing and it moves a 1/4 inch to a 1/2 inch it needs to be replaced. Really I say if it moves that easily with a screw driver or prybar you might want to remove it.
Now to remove the thing put a floor jack underneath the transmission with a piece or two of wood in between the jack and the trans
floor jack the thing up a couple of inches enought to raise it and support it
Next remove the four bolts that hold the mount in. (It was easier said than done on mine and I had to pb blast and heat the furthers top bolt for two days).
Once removed the mount should slip out from the top. If it doesn't raise the jack a little more but not too much
I had a hell of a time putting the new one in because the welded stud bolts weren't exactly true but i made mine fit
Now bolt on the four bolts and don't forget the exhaust support if your truck has one
If the top bolts won't line up lower your jack down a little
Tighten everything up and on the bottom two if the new part doesn't have lock washers or something like that you may want to use a little loctite