I found this on GM's techlink site, thought I'd pass it on.

The 1997-2003 Corvette, or 1993-2002 Camaro and Pontiac Firebird equipped with the 6 speed manual transmission may exhibit the following.

Primary Condition
- Unable to shift out of fourth gear.

Other Conditions
- 3-4 upshift -- hard shift, gear clash, or block-out.
- 4-3 downshift -- hard shift, gear clash or block-out.
- 2-3 upshift -- hard shift, gear clash or block-out.
- third gear hop-out
- fourth gear hop-out.

Replace the aluminum 3-4 shift fork with an iron 3-4 shift fork. The part number of the iron shift fork as of July 9, 2003 is 88996653 and is available only through GMSPO.

The 3-4 synchronizer and 3-4 blocking ring should also be replaced at the same time.