reverse jammed

[yellow jacket]

Have 03 HOengw/6 spd, 285,000 mi, original clutch. . I self installed short shifter kit - truck ran flawlessly until few months ago. After complete warm up and driving 40-50mi, I have an almost impossible time getting into reverse! Took out the short shift, reinstalled stock shift but still have the same problem. All other gears shift fine. I run Pennzoil sync trans fluid - change every 25,000mi. Have no problem with clutch. Any ideas????

Thanks

Yellow Jacket