Yes, I have to agree. It sounds to me like your clutch is simply out of adjustment. It's not dissengaging enough to freely shift gears.
Also, I hear people complaining frequently about Tremec transmissions, but I've had a TKO since 1997 with no problems. It's a strong trans and shifts great. However, if you use any other transmission fluid besides the required GM Syncromesh, it will cause shifting problems and premature syncro wear. So, make sure you are using the right fluid.
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Dan McClain, Editor
The Mustang Works Magazine
1991 Mustang GT - NOVI Supercharged 377 Stroker
1999 Ford Lightning SVT - 2000 Prototype updated by Roush