The adjustment procedure - Adjuster assembly - Accelerator and Cruise Control cables is in section 5E2-131 and 132 of the service manual book one for a 93 vette. This is the section for ASR/ABS service. This is the adjustment for the automatic shift points of the transmission. It sounds like your problem is that the shift lever will not go back far enough to let you manually downshift into 1st gear. I would suggest if that is the case that you remove the cover over the center counsel and look for something that has fallen in the way of the lever going the full length of it's travel:confused
PS On my 93 the rubber inserts were not in the cup holder ash/tray holes. The previous owner must have had the habit of throwing coins, paper clips, etc. in the cup. If you look there is a large notch cut out in the side of the cup. Thats why I say it could be trash caught in the shift mechanism. I dug about 3 dollars in coins from under the center counsel cover. Even so my lever does not move into 1st gear without some deliberate effort. I suspect that it was an intensional design to make you aware that you were going into 1st gear.
I should have been more clear-you are correct, my shift lever will not pull back far enough. Already had everything apart, and found coins, paperclips, etc. Did not seem to help. Stop seems to be adjusted as far back as it will go.