There's no adjustment at the diff end, it's locked in place with some sleeves. The trans end has slotted holes. You adjust just by loosening three bolts, pushing up/down on the trans and tightening. It affects the driveshaft angle, and that's a source of vibrations if it's wrong as any engine swapper knows - or SHOULD know. It might be our other exciting source.
Turns out the new guy at FM has a 2001 with this problem, and it's been shaky since new. Excellent, something to experiment with.
Eureka. I checked the PPF alignment on Kevin's car last night. You measure the distance from the PPF to the (hopefully non-destroyed) frame rails using a straight edge. Spec is 60-72mm for his 2001, the car was at 55. I set it to 66, right in the middle of the range. Easy to do.
Kevin reports that the vibration at 70 mph is dramatically improved. Based on this, I think we may have found the second exciting vibration...
So you're measuring the distance from PPF to bottom of frame rails in terms of up and down?
Or i guess in simpler/better terms, how much "higher" the bottom of the PPF is from the bottom of the frame rails?
Yup. The diagrams in the manual aren't 100% clear as to the location, but it's either right at the rear of the three trans/PPF bolts OR it's just behind the second. Looks pretty flat there so I suspect you'll get similar numbers in both cases. For us, the exhaust limited where we could put the straight edge so the latter is what I used.
Excellent thank you, i'll check mine this weekend. 
The clutch has been replaced on my car, so i'm guessing chances are pretty high the specs are messed up.
I'll give that a shot, too, I'll have to take the exhaust off anyway to put the shiny new exhaust on.
I just checked the 1991 manual - it gives the spec as 66mm plus/minus 5mm. Same range, but it's an interesting distinction. So aim for 66mm.
If you have aftermarket frame rails or your stock ones are banged up, some creativity will be required. It's easy enough to measure how much the FM frame rails stand off the stock ones and add that to your 66mm measurement.
I do have FM frame rails oddly enough . Mind you, the frame rails on my car are pretty banged up but I think I have a little piece of unmolested frame rail at the back.
I had a similar shake at interstate speeds in the s2k. I tried 3 different sets of wheels and tires that were all balanced at different shops. It finally went away when I got TRM wheels with tires from tire rack. They road force balance their wheels and it hasn't been a problem since.
Well, another reason to check the very simple things first.
Had the shop check the wheels and tires again. It looks like the wheel weights they were using didn't agree much with the surface on the Rotas, so one of them made a bid for freedom. Got the rebalanced wheels back so I'll put them on tomorrow and check if the wobble is gone now.
Keith Tanner wrote: Eureka. I checked the PPF alignment on Kevin's car last night. You measure the distance from the PPF to the (hopefully non-destroyed) frame rails using a straight edge. Spec is 60-72mm for his 2001, the car was at 55. I set it to 66, right in the middle of the range. Easy to do. Kevin reports that the vibration at 70 mph is dramatically improved. Based on this, I think we may have found the second exciting vibration...
Wow! When I rebodied my Miata I had no idea there was a spec on this. I hit a pothole the other day and now the Miata shakes at speed pretty well. I'm sure it's a bent or damaged wheel and tire, but it's interesting this measurement. I'll check it out.
Joey
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