Please note I am not a California attorney and general rules of practice may be different. This is not legal advice and I would advise establishing an attorney client relationship with a licensed CA attorney in this case. That said:
In my practice that is normal although the language should say some thing to the effect of explaining that the check will be distributed upon the executor's receipt of the release. Keep in mind that assuming you agree with the accounting and the distribution, it will be easy to prove that you signed the receipt/release but didn't get the check or cash it. You have to remember that the estate's representative and the lawyer are trying to protect themselves from you getting a check, cashing it and not promising to forego a lawsuit on the estate or refusing to pay back erroneously distributed funds so unless you can both be in the same room to exchange the release and the check at the same moment you'll probably have to do it this way.
This is not legal advice nor intended to create an attorney-client relationship.
This is a relatively common practice. The only other alternatives would be for your wife to (a) drive from Merced to Santa Clara County and pick up the check in person or (b) find a "neutral 3rd party" in Merced who can act as an "escrow agent" who would give your wife the check at the same time that your wife signs the receipt.
But if you don't trust the trustee, before your wife does anything perhaps she should require an "accounting" from the trustee ... bear in mind that an accounting takes time to prepare and the cost of preparing it is paid by the trust - so the fees for preparing the accounting will reduce your wife's check. But it should give her some "peace of mind" that all of the money has been used for "proper purposes".
The information you obtain at this site is not, nor is it intended to be, legal advice. You should consult an attorney for individual advice regarding your own situation.
Your question is not uncommon and neither is the attorney's request. Some attorney's chose to send beneficiaries the receipt before the check. This is to ensure that the attorney gets the paperwork he needs confirming that you received the funds (otherwise, many beneficiaries cash the check but don't send back the receipt). However, he cannot require you to sign the receipt before he sends you the check. If you are local, you can arrange to meet, at which time you can get your check and he can get his receipt. If not, explain to the attorney that you will not sign the receipt until you have the check and that you will be sure to promptly return the receipt to him.
I am only licensed to practice law in the State of California. As such, answers are based on an application of California law to general factual scenarios and should not be considered for any other purpose. The information presented in this answer should not be construed to be formal legal advice nor as forming a lawyer/client relationship. Individuals reading this answer are encouraged to seek independent legal counsel from an attorney licensed to practice law in their state for advice regarding their individual legal issues.