PLEASE HELP 99 tracker problems!!

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