The rear of the Tremec TKX has splines that seem like they would fit into the drive shaft, although I didn't check that before installing the engine and transmission. The adaptor plate fits the rear IRS. The four threaded bolt holes on the adaptor plate are in a rectangular pattern, not square like the back of the drive shaft. You can sort of see this in the picture above or look at my caliper measurements. And for the life of me, why do the pictures keep showing up in a random rotation? Sheesh. I'll call FFR tomorrow, but I agree that it could be the adaptor plate or the driveshaft.