buzzing gearshift stalk

After replacing my back-up- light switch on my tranny, I topped off the transmission with gear oil. I then beagan having a buzzing in my gearshift lever. I have tried the simple stuff(drain and refill trans, check for loose bolts.) There is one thing that I need to replace and that is the rubber boot that divorces the shifter rod from the center tunnel. The boot was already shot before I did the back-up-light switch. What else could it be? I looked at the tranny mounts and with the car jacked up I can see an air space between the rubber part and some huge washer bolted to the body. I tried jacking up the tranny one time and stopped when I saw how much it moved before the body lifted slightly. I was going to go to the mounts next. The vibration is only at the gear shifter. It is only buzzing in the mid gears. I can jimmy the gear shift or hold it and it stops. Any ideas to help is apreciated

Spencer - I had the same problem on my 79 SC after installing a Weltmeister short shift kit. I was able to solve it by adjusting the shift rod linkage at the coupling on the tranny (just behind the rear seats.) This is a very finicky adjustment and you have to play with it for awhile. Adjust and test, adjust and test, etc. The procedure is well discussed in the tech articles here on the website. Check them out>