Nick,
I just fixed this problem on my '92 SHO with 188k on it. I hadn't missed any shifts either, but there was a lot of play in the gear shift side to side and I would get a "rattle" during 2500-3500 rpm range.
When you get under the car, you'll see two rods--one is the fixed stabilizer rod and the other is connected to the shift input shaft of the transmission.
As the other posts indicated, make sure your shift input bolt/nut assembly is on tight. If it is and you can rattle the two linkage assembly, you'll need to replace some bushings.
You'll notice the two-swivel assembly just aft of where the rod attaches to the shift input on the tranny is not meant to come apart. I took mine apart and got some new nylon bushings from Lowe's and put it back together with grade 8 bolts, nylock nuts, and threadlocker.
You would not beleive the difference in the feel of the feel of the shifter. Absolutely no play and very precise. I was well-pleased. thumb
PM me and I'll tell you the specifics and see if I can tell you exactly which bushings etc I used to fix this. It only cost a few dollars, but the payoff is tremendous! thumb
VR
Cosmo