Orders for new big rigs plunged and inventories of unsold trucks soared to their highest levels since just before the financial crisis, as uncertainty about future demand and a weak market for freight transportation weighed on truck manufacturers.

About 67,000 Class 8 trucks are sitting unsold on dealer lots, after sales in March dropped 37% from a year earlier to 16,000 vehicles, according to ACT Research. Class 8 trucks are the type most commonly used on long-haul routes. Inventories haven’t been this high since early 2007, said Kenny Vieth, president of ACT.

“It boils down to, at present, there are too many trucks chasing too little freight,” Mr. Vieth said.

Companies that placed large orders in late 2014, only for customers to move less freight than expected last year, are reluctant to buy more vehicles now, analysts said. Online freight marketplace DAT Solutions reported last month that spot market rates for dry vans, or the box trucks that are ubiquitous on U.S. highways, fell 18% between February 2015 and February 2016, an indication of weak demand.

“Fleets are being very cautious in the current uncertain economic environment,” wrote Don Ake, a vice president with FTR Transportation Intelligence, which reported similar order numbers for March. “Freight has slowed due to the manufacturing recession, so they have sufficient trucks to meet current demand.”

Aaron Tennant, owner of Simplex Leasing Inc., a trucking company in Jamestown, N.D., said that last June, anticipating market growth, he placed an order of 115 new Navistar International Corp. trucks to replace 75 trucks and expand his fleet to 245 vehicles. But that growth has not come.

“Once this order is complete, I’m probably not going to consider buying any new trucks until at least October or November,” he said. “It’s definitely coming from caution. The market has softened in the last year.”

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Stifel analyst Michael Baudendistel wrote in a note Tuesday that the backlog of Class-8 trucks appears to be about six months, and said that truck and truck component manufacturers like PACCAR Inc., Navistar and Meritor Inc. are likely to see further pressure on their share prices and earnings.

“We believe that the companies most closely tied to the Class 8 vehicle production cycle will likely have a difficult time exceeding expectations this year,” Mr. Baudendistel wrote.

Write to Robbie Whelan at robbie.whelan@wsj.com