I have a 97 Z71 Silverado, 5.7 that acts like it's puling a 10 ton trailer. When this happens the tach and spedometer go to zero. From a dead stop you can achieve 5 mph by holding the throttle wide open. HOWEVER, If I load the rear end with rocks till it drops the truck runs and acts file. I'm suspicious of the transfer case or a sending unit if there's one on the Tcase but it's push button 4WD so maybe a short in the wiring??? It's not necessarily the weight but the repositioning of the rear bumper (closer to the street) that makes this problem disappear. HELP!