We bought a 1967 house in November and installed a refurbished top loader washer. It immediately backflowed water from the waste pipe, so we had the pipes rotorooted. That fixed the bathroom (slow drains) but not the washer; tech then said the 1.5" pipe couldn't handle the outflow, and to duct tape it. That's no longer working. After 7-10 seconds of draining, the water starts flowing back out of the pipe and down the wall. :(
A plumber took a look and recommended replacing the 1.5" with a 2" pipe and to bring the grade up to code (currently has no grade, totally horizontal through the crawlspace).
And... could adding something to the washer hose restrict the flow (nipple, washer) work? Or will that kill the washer?
Currently, I'm turning the washer on and off during the drain cycle (only doing Small loads) to prevent the backflow, which probably isn't good for the washer either.
Thanks!

Properly sloping the drain will make a huge difference. Also check to see that it's properly vented as well.