Unless your running a booster pump stick with a 75gpd membrane (if it makes enough water for your needs). If you run higher gpd membranes you'll see slightly lower water quality coming out of the membrane. At 2yrs I would just do a membrane they aren't that expensive.

In my setup I run a pleated pre-filter into a 2mic spun filter into a carbon block then to the membrane and finally 2 stages of di resin. I run the filters, membranes and max cap resin from spectra pure. \

You said there isn't much di resin left? I have never seen my exhausted di resin actually look like it was being consumed. I just see the tds go up. Mine doesn't go up much because I am feeding the resin about 2ppm from the membrane. If I don't see 0ppm from the resin then I know it's about time for a change. Make sure you let it settle in before you take your tds readings, tds will be higher right after startup.