03 grand cherokee, just replaced the mc in my 03 gc limited 4.7 (thought this was the cause of brake pedal issues) have now narrowed problem of pedal nearly going to floor (no brake warning/abs lights). This only happens when going from reverse/stopping then going forward. Brake pedal goes almost to floor and then braking is normal after pedal returns.
There are not so many options as where the fluid goes to when you almost floor the pedal. Or the ABS system is letting fluid to return, or there is some mechanical problem on one of the brakes, which lets the piston go out more in certain cases.
JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.
I am assuming that you have looked at both front and rear brakes?
Make sure none of the calipers are loose (though I am sure that you would have noticed that by now)
Another thing to look for is to make sure that you have all the little hardware that is part of the braking system..like any retaining pins or pad clips.
The way that you are describing it sounds like there is something loose in the system that is waiting to catch again when you change directions (even though there are probably about a million other things that it could be)
Keep us updated!
EDIT: Have you tested the booster? I know that it is a problem that does not always happen unless a certain situation, but it could be something worth looking into.