My guess is either the torque converter or it's the transmission is slipping. My Tahoe did it when we were on road trips. It would shift REAL hard from first to second. Also I had a Isuzu Trooper back in the day and it sounded almost exactly like what you were saying, but it was the transfer case from the 4 wheel drive trying to kick in when it wasn't supposed to. Definatley the trans though!