I noticed yesterday that the tailpipe had rusted completely off of the muffler. Took that off today. Muffler has some rust, but doesn't look like it's ready to go at any minute...How long would you say that it can be safely driven as is? (I plan to put on a new muffler asap)

 

If the exit is past the cab you can go indefinitely. If not, you are in danger of fumes entering cab.

 

Years ago I had the same situation on my F150. Ran it for some time, however wound up melting one of the parking brake cables in doing so. I would probably get it replaced sooner rather than later.

 

Exhaust gases exiting under the truck will also increase the development of rust.

 

If you want to run it the way it is, I'd recommend you at least put a turndown on it if there's anything left at the back of the muffler to clamp it to.

 

Agreed on all of the above