Much confusion.
First, you should probably get IRS publications 590 and 575.
www.irs.gov or 1-800-TAXFORM.
Next, you refer to 20%. This is probably the amount of federal
withholding. It is only an approximation of the tax, not the tax itself. For any [pension, profit-sharing, 401(k), etc.] plan, whatever amount is distributed to you will generally be subject to federal taxation in the year it is paid, unless rolled over to an IRA. State tax as well. It is unlikely that a disability status will change that.
Get thee to a tax advisor, preferably one who has some familiarity with pension plans.