Greedy

Somtimes when I put it in reverse and step on it, the whole truck shudders really bad and does not move. If I just let it move without giving it any gas, it is fine but as soon as i gas it a little, it happens. If i put it in drive, go forward a bit, and then reverse again, it usually goes away. is this a major problem or just a fluid/Maintence issue? By the way I have a 3.4L auto with the modified valve body.