Have you checked the linkage adjustment?
Proceedure:
1. With engine stopped and parking break on, place the transmission selector lever in D or OD whichever is on the indicator. and hold against the stop (with a 8 pound weight or have someone hold it for you).
2. Loosen the shift rod adjusting nut (at the link to the transmission).
3. Shift the manual lever at the transmission into the drive position, by moving the lever all the way rearward, then two detents forward.
4. When the selector lever and manual lever are in the D position tighten the nut tp 12 - 18 ft lbs. dont allow motion between the stud and rod while tightening
5. Operate the shiftlever in all positions to make ssure it detents into all gears. Readjust if necessary.
You should never adjust in anyposition but D or OD.
Hope this helps, if not you probably need a rebuild or new C6

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