Is it possible to install 2x4s on their side over the block with 1.5" rigid foam insulation?

i have 2 bathrooms with fiberglass tub and shower s they are old but but still look ok i want to keep them and just put new doors on them and get this place rentedthe ploblem is when you stand in them they seem to flex and make a slight squeek noise seems kinda flimsey is there and way to bulk them up i was thinking of getting to the undersidethrough the out side acess doors and spraying that foam or something under there to give it some more support ,,, any ideas ?

I think I paid $0.45 per sf last time as compared to around $0.25 sf for foam.

Currenly the house is a 4/2/2 with a steep roof that is foamed.

I am getting about 500/pier and depends on the sqft for foam injection.

As @Kenny Lee and @Thomas S. have advised, connect with a financial planner (fee-only... can't insist this enough) to (A) understand your options, (B) why they are important and (C) how they apply in your case. 

Closed cell spray foam has a built in vapor barrier.  

Today's lunch break was a door that sticks in the frame and I had to shave a 1/8" off the door before an upcoming check-in. 

Consider changing your garage door opener code as a security precaution.Service your lawn mower and yard equipment (blowers, edgers and trimmers).Clean your faucet aerators and inspect your plumbing for leaks.Clean and sharpen your garbage disposal by running a tray of ice cubes and depositing a cleaning (foaming) product into the disposal.Inspect or have tested your home fire extinguishers.Replace smoke alarm batteries.

Also, foam boards are below.