You seem to be describing a very specific feeling that does not correlate with loose mounts but it's probably worth checking the front diff mounts anyway. The way you're describing it it's almost like the rear axle is torquing causing the squat feeling before it goes. But I know you said you checked all of the mount points and bushings back there.
Might be worth setting up a GoPro under there to see what's happening when you take off from a stop.
Might be worth setting up a GoPro under there to see what's happening when you take off from a stop.