ltllis1977

You have been given great advice.  It all boils down to evidence, and you need to review your file to see what is actually in there, which is often different from what the Veteran "thinks" is in his file.  Stuff gets lost and shredded and docs dont always write down what they tell you.  
If you have not filed the 21-8940, your claim will be "administrately denied".  
Its not enough you are unemployed.  
You have to be UNemployable due to SC conditions.  This is different than "unemployable" or even unemployed.  Lots of people are unemployed due to many many reasons..such as the company layed off half its workforce due to budget concerns.    
A doc needs to make an opinion that you can not work due to SC conditions.  Read in your cfile to see if one says that, or something close.  
If there is no nexus, then you need to get one or your claim will likely fail.  

Itllis1977,
What you need to prove that you are unemployable is that you cannot 'secure gainful occupation'.  That is the legal standard and, unfortunately, it has not been well defined by the VA.  What it is not is an review of whether you are able to do sedentary work.  That is a concept from Social Security law that has never been formally adopted by the VA.  Nevertheless, you see denials from ROs on this basis.  
 
What VA must consider is your educational and vocational background and how your disabilities affect you.  The key here is that the standard is a subjective standard--i.e. it is based on your situation not the average veteran standard, which is used in all other situations.  Your job at USPS was marginal employment and does not count against you.  If anything it shows that even when you try to work you cannot.  Furthermore, even if you are looking for work that does not preclude IU.  If you keep getting jobs and losing them this would be further proof of your claim.
If you are interested in working then I would recommend going through VA voc rehab.  This will result in one of two situations.  One VA trains you/ helps you find a job where you can work and then you can reenter the work force.  At that point, you don't need iU.  Two, after a good faith effort on your part it is determined that you cannot work then you have evidence from VA's voc rehab that you cannot work.  That is powerful evidence for your claim.
Good luck
 

I sure hope you apply for SSDI and not SSI.  Always remember that SSDI (Social Security Disability Insurance) is for people who paid in the SSA system and become too disabled to work and file for disability insurance.  SSI (Supplemental Security Income) is for people who are low income and did not pay in the SSA system.  Since you are already receiving a 70% combined service connected benefits, your income is too high for SSI because it is income based but SSDI is benefits that you paid in and earned and it is not income based.  Just wanted to point that out.