This is what I know, after all I had a hand in the transportation of almost 200 of those MRAP's to Afghanistan some years ago.
It is highly unlikely that they were carrying any thing more than 5 MRAP's as the addition of the 6th would have required substantial ballast in the forward of zone F (the upper deck barrier) or a significant amount of ballast fuel in the center tank.
That was the primary reasoning behind the military scrubbing any further trips with 6 as they did not have sufficient cargo to transport at the time and carrying 25000kgs of ballast fuel plus was not considered feasible at the time.
The key question here is what else was being carried as with with only 5 MRAP's the aircraft would have been outside of the aft center of gravity envelope so generally the support equipment was used to balance the aircraft to within trim, occasionally additional ballast was required.
IF the MRAP's were secured in the best possible manner the were loaded on top 22 military pallets joined with a T2 coupler and shored on 4 specific build shoring blocks that distributed the weight evenly and then secured with chains to the pallet. Locks were then relocated within the Cargo Loading System for the best possible alignment in the center of the aircraft both forward and aft of the pallet. They were then treated as floating pallets and secured to the aircraft structure using straps. The number of straps I don't recall offhand but it was close to 50 per vehicle.
Using this method of securing them would render the probability of any load shift remote at best.
However I am not sure if this was the method used to transport the vehicles as some carriers opted to utilize the industry standard 16 or 20 foot pallets and shifting the MRAP's towards the aircraft center line to allow space for other cargo.
There is also one factor that no one that I have seen mentioned yet, on the newer type of converted freighters they have done away with the "Big Wheel" type of Power Drive Unit that were used in the past and moved to the smaller tray mounted retractable units. The only problem with those is that IF you have a shift in load those units do not offer any friction and the cargo is free to roll in either direction.
my 2 cent's worth.