When you drop a truck 6″ you can expect the factory shocks not to work. I know, I know there are a few lucky guys who say there factory shocks work just fine compressed 6 or more inches, but I have never been that lucky. The new Monroe shocks blew out within a few miles. Not like it mattered, they were bottomed out anyway.
Typically it takes a while to figure out the best shocks for your application. Thankfully, now a days we have the internet and in this case a Mazda Facebook Group. I simply posted a question asking what shocks would fit in the rear. The common answer was VW Bug air shocks.