When reservists get put on active duty, how do they make up the pay difference?

My suggestion would be look at becoming an officer. By the tie you make Captain you would be an O3-E and you were activated chances are that your pay would be fairly close. As an officer my pay was actually more at the end of the day due to all of the tax free stuff then my civilian job even thought I technically made more money as a civilian. It is an eye opener when you realize how much of your paycheck goes to taxes when you have a civilian job over how much you keep, such as BAH, BAS and in some states no state tax on mil pay.

Otherwise like others have said you are screwed. Your only choice would be to put money aside in savings and have a plan to scale down your expenses if you were to get activated. If you are renting a 2000 dollar a month super hip loft in pioneer square or Cap Hill you may need to unrent that and move your family to Spannoway while deployed.

It wholly depends on the company you work for... For example, Varen Technologies pays the difference (or did when I worked there), others leave you on your own. Speak with your Mortgage company, etc. and explain the situation and see if they can work with you. Most can, and I guarantee you they have encountered this situation before...

Deliver pizzas at night. Reserves is a sacrifice for many.