Soviet forces are beginning to withdraw from eastern Afghanistan, where anti-communist guerrillas have taken several posts since the Geneva accord was signed, Western diplomatic sources said Tuesday. The sources, speaking in Islamabad, Pakistan, on condition of anonymity, said the Red Army also has begun transporting wrecked vehicles back to the Soviet Union, apparently to prevent them from being salvaged by the insurgents. The estimated 115,000 Soviet troops are to start leaving Afghanistan on May 15.