My husband and I are both Public School teachers.

This is both of our second years teaching. We have both been paying back our student loans for two and a half years under the standard payment plan.

We recently sent in paperwork to get us both transfered to IBR plan (per recommendation of coworkers).
We were also told to apply for PSLF

We start grad school now in January—Do our loans go into deferment while we are in school for a year and a half?

Do we apply for PSLF before or after grad school?

Do they count back payments?

My husband is through Navient and I am through Great Lakes.

I found this post from Heather which should answer your deferment question. I believe you can opt out of the deferment for the loans that are in repayment status.

http://askheatherjarvis.com/forums/viewthread/9500/

You can’t technically apply for PSLF until after you’ve made 120 payments and your loans are eligible for forgiveness. You can periodically submit employment certification forms which they will verify how many qualifying payments you have made thus far. (strongly recommended, as you don’t want any sudden surprises). When you send in your first form, your eligible loans will transfer to FedLoan Servicing from navient or Great Lakes, and they will be your new loan servicer. (they are the only one with the authority to monitor the PSLF program). They will move like a glacier in processing your paperwork, (took them 6-7 months for mine) but once you are set up, the wait time is much less for subsequent certification forms.

Past payments will count if you meet the 3 criteria of having Direct loans, being in the right repayment plan, and working full-time at a qualifying job. Be very certain that your loans are in the direct loan program. if you log into the nslds website and look at the loans to your name, they should all have the word “Direct” in them. Loans that show as Federal Stafford Subsidized or unsubsidized are probably in the FFEL program and not the Direct loan program, and will not count unless you consolidate them into a Direct Consolidated Loan.