WASHINGTON, D.C. -

Two of Chrysler Group’s most popular models — the Ram 1500 and the Jeep Grand Cherokee — are now the subject of separate probes by the National Highway Traffic Safety Administration’s Office of Defects Investigation.

Nearly 337,000 vehicles are included in documents posted on SafeCar.gov; announcements that cover the 2009 and 2010 Ram 1500 and the 2012 Grand Cherokee.

The larger of the two investigations involves an estimated 230,000 Ram pickups. ODI officials said they have received 12 complaints alleging rear differential failures resulting in rear wheel lock-up in these units.

The complaints allege failures at various speeds with eight of the incidents reporting failures at speeds of 35 mph or greater.

The agency indicated that four of the complaints involving higher speed incidents allege that the failure resulted in a loss of control (such as a vehicle spin, skid or yaw), including one resulting in a crash into a concrete barrier.

Furthermore, ODI noted one complaint reported that the failure was caused by loosening of the rear differential pinion nut.

The agency added that most of the complaints allege that the failure resulted in drive shaft failure including one that reported the detached shaft end punctured the fuel tank.

Meanwhile, a separate ODI file has been compiled on an estimated 106,803 Grand Cherokees.

ODI indicated it has received one complaint of a power steering return hose failure in a 2012 Jeep Grand Cherokee vehicle. The agency also identified additional incidents of power steering hose failure in the subject vehicles in field reports submitted by Chrysler in its early warning reporting submissions.

Officials fear power steering fluid may be ignited by hot surfaces in the engine compartment.

ODI also received two reports alleging engine compartment fires while driving in the subject vehicles. Both complaints were received within the last month and allege severe fires resulting in total vehicle loss with the cause as yet undetermined.

The agency added one of the complaints reported fluid dripping beneath the vehicle during the incident.

In summarizing each investigation, officials said, “a preliminary evaluation has been opened to assess the cause, scope and frequency of the alleged defect.”