Miata vibrates/shudders under load

My '97 (with nearly 200k miles) has started intensely vibrating/shuddering at a variety of different speeds and gears. It happens mostly at sustained or moderate throttle, and stops if I let off the throttle. It also doesn't seem to happen if I am accelerating aggressively.

I am thinking it could be the clutch slipping. Any thoughts before I take it in to have the clutch replaced?

I doubt it's the clutch (most trouble should be under heavy load)...check the motor mounts.

Seized or worn u joint?

Just throwing out some ideas. Check the rims for mud, dirt, or general gunk. Did you have a wheel lock up recently? If so possible flat spot. Could some of the wheel weights come off?

I don't think it's the wheels, the second I put the clutch in, it starts to go away.

Sometimes there is a "clang" noise when the I put in the clutch, so I'm thinking it is the U joint after seeing some videos. I'll check the motor mounts too.

Sounds like I have to replace the whole drive shaft, correct?

Anyone know if this is a good place to buy one? http://www.wholesaleimportparts.com/Mazda_Miata_Driveshaft_1997.php

I know this is a good place, and it'll even save you some money :) https://www.flyinmiata.com/18-rebuildable-driveshaft.html

The factory joints aren't replaceable, although some shops claim they can do it. Every Miata driveshaft has flat spots in the U-joints, so be prepared for that. But if you're getting a clang, it's pretty far gone.

Thanks Keith, I'd much rather buy from you guys.

What do you mean about being prepared for the flat spots in the U-joints? Something I need to know when I go to replace it?

Edit-nevermind, I read the product description. Yeah, mine is pretty shot, I reckon.

I think the car has the Torsen diff, does that affect what part I use?

Torsen makes no difference, you just need to know 1.6 vs 1.8. Hint: 1.8 if you have a Torsen

The new one won't have flat spots, but almost everyone who removes a used driveshaft notes that they have flat spots. So a bit of notchiness is normal, a lot is not.