LEAD: A main nut on the rotor shaft of a military helicopter was replaced just two days before it crashed, killing 15 servicemen, military officials said today.

A main nut on the rotor shaft of a military helicopter was replaced just two days before it crashed, killing 15 servicemen, military officials said today.

Search crews today recovered the last of the bodies of 4 Air Force Reserve crewmen and 11 Army Special Forces members from Fort Bragg, N.C., from the charred wreckage of the CH-3E helicopter.

The Vietnam-era helicopter went down in flames Sunday night 25 miles northwest of the Davis-Monthan Air Force Base, a base spokesman, Capt. Carlos Roque, said. A temporary morgue was set up at the base in preparation for the arrival of an Air Force forensic team to complete identifications of the bodies. Craft Flown 3 Times Before Crash

Navy Comdr. David Thomas, a spokesman for the Pentagon, said today that more than 300 CH-3E's and similar helicopters belonging to the Air Force, Navy and Coast Guard had been grounded for a week ending Friday so a nut attaching the main rotor head to the helicopter frame could be inspected.

Air Force Lieut. Col. Darrel Hayes said the helicopter that crashed had been inspected Friday and the nut replaced. It had been flown safely three times after that work, he said.

Officials at Davis-Monthan grounded the base's five remaining CH-3E helicopters Monday.

Colonel Hayes said the team investigating the accident would try to determine if there is any reason to ground similar helicopters but said there was no indication so far of such a need. Four of the Dead Identified

The crash took place after the helicopter refueled at Davis-Monthan while ferrying the soldiers from Fort Huachuca, 80 miles southeast of Tucson, to a gunnery range near Gila Bend, 125 miles northwest of Tucson.

Four of the victims were identified as Lieut. Col. Lawrence M. Rolle, 41 years old, of Phoenix, commander of the reserve squadron and co-pilot of the helicopter; Maj. Donald D. Thomas, 42, of Tempe, the pilot; Master Sgt. Malte Breitlow, 45, of Tucson, and Tech. Sgt. William E. Slaven, 37, also of Tucson.

The names of the other victims, all soldiers attached to the 3d Battalion, 5th Special Forces group at Fort Bragg, were not released.