I'm not an expert on this and it is a huge topic with all sorts of opinions. Here's what I know...
Vibes on cardan CVs are not uncommon, and the internet is choked with this same discussion for all vehicles...mostly the full-time 4WD Jeep guys. I would give Tatton a call and perhaps he can shed some light on the issue. If any of you find some useful info...post it up. Cardans do not have the inherent vibe-cancelling features that the stock 6-ball CV has (which is why GM uses them). Because the cardan is a full-mechanical connection...they tend to transmit more vibrations. I have experienced vibes on previous driveshafts and when I did have them, it felt more like a low-frequency "hum" rather than a vibe. Like ABN and CGAlpha...it seemed to come & go at certain speeds and under certain loads. Mine was most noticeable in 4th gear (with the torque converter locked-up). One thing to keep in mind, is that ENGINE rpm and DRIVESHAFT rpm are two totally different things. ENGINE rpm will pass thru the same "vibe-range" in every gear. So, when you say you have vibes at certain ENGINE rpm...I think the issue might be nothing more than motor vibes getting transferred to the cabin via the driveshaft. This type of vibe would occur in every gear, at that same motor rpm. It would be very similar to exhaust drone, and how it behaves. It might also be "load induced". Conversely, DRIVESHAFT rpm will not pass thru a vibe-range in every gear. It will only occur at a specific vehicle speed...and will only be present at that speed...regardless of gear.
One quick thing to try is shimming the front diff pinion up or down with some washers to see if it has an effect. I would try a 1/4" thick washer (or washers)...that should be enough of a change to register. Not sure if this would do anything but, u-joints and drivelines are mysterious creatures, and I've heard that a pinion angle adjustment can make a difference.
Tatton says he balances these shafts to 100 mph, which makes no sense. It needs to be balanced to an RPM, and not a MPH. MPH can vary from vehicle to vehicle depending on gear ratio and tire size. EXAMPLE: A car traveling at 100 MPH with 2.73 gears will be spinning the driveshaft at a vastly different rpm than the same vehicle with 4.56 gears. So, I don't know where the 100 MPH balance comes into play, and how he arrives at that number. FWIW...my Tatton-supplied 1310 shaft is 100% vibe free. None. Zero. Nada. However I am running a solid axle, so its not really an apples-to-apples comparison.
CGAlpha concerns me when he says he has a "grinding" sound. I have never had a grinding sound on any shaft except one that is total junk. I cannot imagine what would be grinding but, whatever it is...it ain't good. I would park it until you can find out.
