generally a worn clutch loses reverse then first. But it could also be one of the hydraulic cylinders. There is a tool you can make to check clutch wear, someone here will chime in with a picture.

edit: see post #3 here

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)