I have a single lever tub and shower valve and the diverter lever is on the trim plate, each time I turn the diverter the valve leaks a little. There is a nut at the back of the valve going into the main valve... can I tighten that with a deep socket? Or remove it to replace the seals?

That is probably the way to remove the diverter valve. Show us a picture.

Here is a pic

Yes, remove and rebuild diverter. What's the brand?

Its Pegasus, from Home Depot... so it could really be any brand. I called them for help, but I did not keep my info with the model number so they weren't very much help. So a deep socket should do the trick? I'll work on it tonight and let you know

Probably need a set of tub faucet wrenches.
https://www.askmehelpdesk.com/members/hkstroud-albums-+pictures-picture530-deep-faucet-wrenches.jpg

Pegasus carried by Home Depot.
Most Pegasus faucets are manufactured by Globe Union Industrial Faucet Company of Canada , China, and elsewhere. Their brands, besides Pegasus and Glacier Bay, include Danze and Gerber.




Hope this helps, Tom