Based on the below news report,  August of 1950 was the very first crossing of the Naches Trail by the Yakima Ridge Runners.  This information resides on the Nachestrail.org website.

Something I didn’t know about this first crossing was that the jeepers had to use winches to drop their jeeps over the cliff on the west side, just as pioneers had done almost 100 years before (well, they used freshly made cowhide ropes instead of winches).

Note that in transcribing this article into PDF form, the transcriber couldn’t identify everything, so there are a few uncomplete hiccups in the text.

Though the article includes no images, only 9 months later many of these same folks and their jeeps would be photographed by Life Magazine playing in the Yakima Valley hills, sagebrush and mud (included below).  You can see all the pics from that here.

Jeeps Follow Trail Carved by Naches Pass Emigrants
08-15-50
By Ted Van Arsdol, Yakima Daily Republic

Ninety-seven years after the first wagon train crossed Naches Pass through the trackless Cascades, 11 Yakima jeep drivers, members of the Ridge Runner club, have completed the same rugged journey.

The jeep caravan returned here yesterday after following the forest-grown trail across the mountains. The jeep riders lowered their vehicles by chains and a winch down the steep face of a cliff on the west side. It was on this same cliff that pioneers of 1853 were forced to dismantle their wagons and lower them by ropes.

W R. (Wally) Klingele, one of the Ridge Runners making the trip said. The group could still see traces of the old wagon trails and the ancient cuts on trees that probably were made by the pioneers. He said there were also grooves on the west side cliff made by the sliding covered wagons.

First Four-Wheel Repeaters
“I believe our trip over the old road was the first one made by four-wheeled vehicles since the pioneer trek,” Klingele said. He had heard of motorcycles going over Naches pass, but doesn’t believe they went down the face of the cliff as the jeeps did.

Members of the Ridge Runners who made the trip were Chet Thompson, Dale Rohn, ___ Golsh, Gordon Buckley, Harlan Beckett, Lyle Christopherson, __ King, Bob Schultz, Pat Mullins and Klingele. Roger Gervais, jeep club initiate, also made the mountain trek.

The group had gone into the [?pass] one week ago from Timothy meadows to Government meadows. They left Saturday from the mouth of American river, went about seven miles by road to Jungle creek camp and then journeyed on by jeep trail to Timothy meadows.

Had to Carve Road
They camped overnight at Timothy meadows and started at [??] am Sunday on the trip through the woods. It took 11 hours for the caravan to cover the 10 miles to the pioneer cliff. The drivers had to cut and clear a road using shovels and axes.
To reach the bottom of the cliff on the other side, the Ridge Runners hooked one jeep to a tree by a chain, and the man in the jeep used a winch to lower other vehicles down the cliff.  [?] used a crank to unwind the winch, while the other Ridge Runners worked with the jeep being lowered to make sure it didn’t go astray.

The 150 foot dropoff was steeply sloped but had a number of ledges. It took 2 1⁄2 hours to lower all the jeeps. The toughest part of the trip came at the end. Yakima’s jeep club spent eight hours covering a mile and a half of the most rugged sort of mountain terrain. The foliage was heavy and many fallen rocks and logs hampered their machines.

After 21 hours on the trail the Road Runners reached Hines forest camp on the west side at 3 yesterday. They covered just 14 miles from the end of the trail at Timothy meadows to the forest camp.