I have a 2002 Z28 w/ 34K miles. 1st - 4th seem to work fine, but when I shift to 5th, it makes an unhappy noise ( like marbles rolling around ) and feels like there is resistance. It will go in but then pops out into neutral after 2 or 3 seconds. It won't even go into 6th at all.

I have some mystery after market shifter in my car and wonder if that could be the culprit?

Side Note::
The last time I drove it before today, I raced someone on the highway. I did a down shift into 3rd at around 70mph and went threw 4th and some of 5th. Thats the last nasty thing I did to the car, and it behaved fine. I didn't hear any noise or grinding until I decided to take it for a drive today, so wtf????

Any help would be much appreciated.

 

Sounds like an issue with the 5/6 shift fork and possibly the keys in the syncro assembly .