I am trying to put in a 36" standpipe 9 foot length 2" drain for a washing machine. I have a 2" trap and havethe run angled almost 9" in 9'. I know that's overkill. My problem is that the drain line has to reduce to 1-1/2" to T into the main drain before it drops into the main to the street.

It is backing up. So I gather I need a vent next to, after the trap, but what do I do about the drain size reduction without a major overhaul?

this same washer was plumbed into another 1-1/2" standpipe but was 4" from the drain line