rear end shaking

Got a problem with my 90 rs 305
Everyonce in a while when car shifts from first to second gear the ass end feels like its gonna shake itself loose. only does it for a split second. It always hits second tho. Never does it with any other gear. Shaking doesnt happen every time only about 1 in 6 or so. no pattern as far as i can tell (hot / cold , high rpm /low). Only time in always never does it is when i let it idle up into second gear (idle is way to high as a side note, havent had time to tune it down)
any thoughts and suggestion, i dont want to take off for work one day and have half of my car stay in the driveway
: P

How long has it been doing that? When did the idle get that high?

When you do get the idle turned down, computer controlled cars can not just have that idle screw turned down. There is a procedure that you have to follow. Not sure if you knew that, just in case.

do you have a manual or an automatic? im thinking the problem is in the tranny not the rear...

"when he shifts from 1st to 2nd gear" i think its manual

"when the car shifts from first to second"

i say its an automatic

as far as turning down the idle theres usually a screw that controls the idle air circuit that you can turn to adjust it. not sure where it is on tbi motors though.

could the gears in the rear possibly be skipping or something like that.

it could be, but im thinking that if it were, the shaking would be constant, not just during the 1-2 shift...