3rd gear synchro shot?
 

after a little bit of hard driving, 3rd gear grinds pretty bad when shifting out of 2nd at high rpm (above 4000). if i double clutch, it does not grind. my clutch is slipping pretty bad, but i don't think thats the problem. is my synchro shot? if it is, how much to get that fixed (there's no way i'm going into the tranny myself)? would it be cheaper if i were to remove the tranny myself and then give it to a shop to fix? how much for a new custom tranny?

o, and the problem only happens with 3rd gear.

hmm... well you coulld slap a newer one in there if ya wanted...that may be a nice cheap fix

a newer KA tranny? do the s14 trannys line up with the s13s?

If you have a DOHC KA motor (most likely u have DOHC since u have a 91), then yes the S14 tranny will line up and fit in perfectly.

HOWEVER... If u have an SOHC then no, it wont line up at all.