My guess is that if there is vibration enough to throw it off the stands at 60mph it will be obvious long before then - hence the need for very gradual building of the speed and jumping out of it at the first sign of tremors.It doesn't sound scary, it sounds like tempting fate. Cars on dynos are strapped down...
Doing this in a fwd manual trans vehicle with relatively little torque is one thing. Trying it in a 3 ton V8 auto truck is something else entirely. If your burb slips off the stands at 60 mph I promise it won't stall.
I'm not dismissing your concern - it is valid - just advocating an approach that makes it viable but not necessarily dangerous. At the first sign of any tremor - the test is over and successful. The problem is at the rear. No vibes and there's nothing to make it jump if everything is done gradually and smoothly. No sudden stabs at the throttle, no braking - just let it run down on a closed throttle (or stick it in neutral).