Hi Jessica,
I haven't been on in a while but when reading this post recalled a somewhat recent change in legally permissible holding times on checks by law. The Federal Reserve under Regulation CC (you can google this) sets out very specific limits on holding time. Although the term used is bank, the regulation defines "bank" to include certain insured credit unions, or ones eligible to be insured. The permitted hold times are limited, and depend on whose check it is, whether it is local (in the same banking area as defined in the regulation), etc. Here is a link to a description of the regulation
http://www.federalreserve.gov/pubs/regcc/regcc.htm.
It may also help you to know, if you can, when the payer's institution actually PAID the check. It was probably significantly less time than the "hold" because most items are now cleared electronically.
It may make a more persuasive argument for retroactive reinstatement of your insurance, if you can show that your deposit was timely and but the credit union did not follow federal regulations.
Also if the CU is continuing to hold your deposits, then knowledge of regulation cc may help. If the institution somehow is not covered by regulation cc, you may want to consider moving your account to an institution that is covered. . Feel free to PM me if you want.