Are the bed rails on our trucks to weak to mount a spare tire carrier that goes from rail to rail?

 

What year? How big of tire?Tailgate helps to add rigidity. Without some reinforcement I would say it probably wouldn't fly.

 

It can be done.
I used a 4Runner rear swing out tire carrier.
There's a re-inforcing bracket behind the sheetmetal in 4Runner.
I chopped it out and trimmed it to fit inside the truck's rear corner.
I had to chop the latch as well.
It works nicely. That's a 33x12.5 on a 15x10 steel rim.
The tired doesn't shake or jiggle at all.

The tail gate latches just fine.
The only time the tailgate didn't latch properly was when I had it parked sidehill on a steep slope and had the swing-out open.
I only had to close the swing-out a bit and then close the tailgate.

 

Mine is a 87 and I don't plan on mounting anything bigger that a 31" tire.