When I switched to a Shep tranny and a new aftermarket clutch, I developed this exact problem. It had nothing to do with tires for me. Oddly, I don't get any P0300 codes.

 

This is interesting. I wrote what I think is going on but it is just a theory, but a theory with some evidence to back it up. I mention tires because I first encounter this problem when I replaced the OEM tire at about 15K. I then picked up a set of OEM tires when Tire Rack had them on clearance. After taking up space in my garage for several years I have them mounted and the 5th gear vibration is 99% gone. This with an '03 transmission with 110K that has never been touched. So, my suspicion is that the factory mules were tuned on the OEM tire and if true that could explain why this problem wasn't detected before production.

You've encountered this problem later in the life of your vehicle so what is your take on what is going on?

 

Hey guys it's been a while since I opened this thread, this is what I've found the issue/issues are. Yes most people are right 2000rpm around town is not ideal, however when I posted this years ago I should have mentioned that yes, I was on the highway and mostly around 2800rpm(meaning between 2-3000 rpm) for those who like to argue.... No I wasn't driving low rpm's in 5th. However the hard vibration is an internal issue, I've found after car meets a lot of evo8 owners do have the same issue. Not everyone tho. No worries your Tranny isn't going to pop. It's a minor annoyance until you send your Tranny for a rebuild. Also I've been told that one of my buddies replaced his transfer case and it went away as well. So it can be one or the other. Both a little costly, but nothing to loose sleep over. As far as the p0300 code that in itself is another issue but yes..... It happens around the same time. For me(not speaking for all Evo owners just me) what fixed this was new ngk plugs and new coil packs as we all know after a while the coil packs develop very fine micro cracks in them so I replaced them both with oem replacements and wires of course. If anyone else has found other ways of fixing these problems please let us know. PS- after reading everything BarneyB has been saying he sounds like he's right on the money.

 

Honestly, I haven't been bothered enough yet to dig deeper into the issue. It's annoying to me, but hasn't turned into a severe problem yet, so it's been such low priority. One day, when all of my 'to-do' items are done, I'll probably start looking for the culprit. I just avoid driving at that low of rpm in 5th. I hope it's not a costly issue.