'Grabby' brakes

Well, theres that then.
Ima still leaning towards a too-long pushrod.
But I am also considering a mismatched front/rear system, or as a last-on-the-list-item, a faulty P-valve.
If the rears are still 9 inchers:
Heres how that could play out. Disc brakes are not self energizing. That means they require a lot of pedal pressure(compared to a drum brake). So the factory usually puts a booster on there. Makes it easy for grampa(me). Now the rears are still self energizing. And say they are 9 inchers. It doesnt take a lot of hydraulic pressure to lock those babies up.So when you lay on some pedal pressure, the booster does its job and multiplies your leg pressure. The front may work fine. But if the rear receives too much pressure, it will tend to lock up those little 9s.
Since the rear is destined for replacement, Im going to throw this out there.If you somehow prevented brake pressure from activating the rear brakes,you might learn a valuable bit about the source of your grabbiness.And if its coming from the front, someone has already posted a way to prove its booster(pushrod) related.