If my husband and I are going to separate, should I wait before selling assets to pay off our debt?

Although not a qualified California lawyer, J.M.A.'s advice on this point is correct.  You gain nothing and can only lose if you have to argue that you are entitled to credit in the divorce for debt you paid off before the divorce.  There is law to back you up in that situation, but why give your husband's attorney an issue at all.  If you are sure you are divorcing and want to get out from under the debt, just get your divorce on file and get the assets and debts allocated.  Then pay off the debt allocated to you.

While you may be able to take into account whatever debt you pay off before the divorce, you do not want to begin paying off debt.  Indeed, this debt is likely marital debt.  You want the court and your lawyers to divide assets equally, including debt.  You do not want your husband's lawyer being able to argue that your husband isn't liable or should take what you paid off into consideration.  I suggest not paying any debt off (unless you are going to risk a default or negative mark on your credit) until advised by your lawyer.  It sounds like you both have assets and that your husband is going to be stubborn here.  Hire a lawyer and leave the debt in place for now.

California is a community property state.  This goes for assets as well as debt.  I would not sell your stock or car to pay off joint debt yet.  Although most people believe that everything is split down the middle, in California the court has broad discretion in apportioning marital assets and debt between divorcing spouses. Community property can be divided in any proportion the judge feels is equitable, or fair, after consideration of a long list of factors. Get yourself a good lawyer in all of this.  Good luck.