Please Help Tranny Or Clutch??!!

Hey people.. I have a 99 chevy tracker 2.0 5 speed 4x4... I had to sign up here because i found this on a google search and you guys may be my last hope... im losing my mind with this.. I was driving down the highway.. smelt like burning.. but only for a second. (i thought it smelt like clutch) then it started quiet and quickly became very very loud.. it was a clapping/hammering sound.. gears mashing? clutch somehow? i could still shift.. it was as if nothing was wrong.. cept for that loud hammering!! the only time i dont hear this sound is in 4th gear. Tranny right? 4th gear is drive gear.. no reduction no mashing.. cool.. but if thats it and i am so unfortunate that i will have to do expensive tranny work.. then why the hell is everyone telling me i need to put in a clutch kit??!! i bought the clutch kit today.. its still sealed.. i cant bring myself to go to the machanic (local not dealer) and have him put in the clutch kit.. just to find out it is my tranny and i now have bought a complete clutch kit for nothing and cant return it.. i am told that i can just take the tracker over and he'll know by a test drive (rriiiiigghhhhtt) .. please any help or suggestions as to what this may be or past experiences with this situation would be greatly appreaciated.

Brad.

Well took it in and i is the tranny.. tearin it apart on tueasday.. ill let ya know.. if anyone cares..