Why did the IRS send me a letter asking me to submit W-2's and 1099-r's to prove how much tax was withheld? I had properly entered this already on my TT return.

It's hard to say for sure, but more than likely it is because something you reported on your tax return does not agree with what was reported to the IRS by the companies that filed the W-2's and 1099-R's.  It could also be because the information one of those companies filed in incomplete or has some sort of problem.  Make sure you entered them correctly before sending, if you haven't already.  If you entered them correctly, I would just send copies to the IRS and not worry.  

@green2ski 

Also...double check the values on lines 25 a,b,c of your printed form 1040 to see if they really make sense too.