i know the 5th gear shift fork was mismanufactured on some ludes namely 4th gens. off the top of my head im pretty sure that 5th/Rev are the same fork. so if that is the case then i guess the shift fork is the culprit. you can try to shim it but that is HIGHLY un-advisable. i took my tranny apart and cleaned it out not to long ago, and its not something you want to attempt if you have never dealt with engine building, the tranny is alot more complicated than the engine IMO.
if u are on a crusade...i heard of some LSD equiped trannys like the f20b and JDM ludes that dont have this issue, you can also put those in 1st gear at like 30 if u want. of course you should NEVER downshift into 1st gear anyway.

~kris