Driveshaft RPM Collar Location Question

[Dewdlebug]

I've run my collar on the driveshaft yoke for over 5 years with absolutely no issues. Yes I'm sure the collar moves longitudinally a fraction or two as the rear suspension flexes but it's never been a issue on the data readings. I run a 12 bolt rear and I have never spent the time to purchase a longer rear gear yoke. The 1.5" collars are readily available from Racepak (2 magnet). I looked a few years ago for a 4 or 8 magnet x 1.5" but couldn't find one.
Make sure you set the pick-up location with the car loaded on all four wheels and not while it's on jack stands.