What is my weakest link?

I am, unfortunately, a cautious incrementalist. My vinyl setup is weak compared to the digital side of the house. So which piece would you replace first?

Sony SL-D2 ('70s vintage)
Rotel RQ-970BX
Grado ZF3

Thanks for any advice...
Hey Dwt, are you sure that's a Sony SL-D2? I've heard of the Technics SL-D2 but never a Sony....? I'm pretty new to vinyl but have made the following observations. Hope you might find them helpful:
1) Does it sound good to your ears? If so, make sure everything is set up right, keep it clean, especially the vinyl itself and be happy!
2) It really depends on how deep you want to go into your pocketbook. You have some pretty nice pieces and depending on condition, especially the Grado, along with the other components of your system, may be just fine.
3) If you replace something, I think I would start with the Grado and see if that makes a noticeable improvement. If the table operates well and you don't hear any noise, then I would think phonostage next.
4) If you replace the cart, phonostage, then get ready because you'll likely be looking for a table that will do the former justice....it's cummulative, buddy! And in ascending order usually as far as the pricetag goes.
5) Lastly, I would find someone in your area that really knows vinyl, phono stages and carts and have them come over to listen to yours to give you some observations. Maybe even bring over another cart if they would be kind enough to do that. This would be very helpful in terms of knowing just what to do next. I recommend going to a retailer and asking some questions about who they know that is into the vinyl, analog thing and see if you can't make some contacts. The dealer may be willing to call them and ask on your behalf for a meeting, so leave your contact information.

For me, the discovery of what music sounds like on vinyl using even "good" components has been an epiphony! I now spend hours on end just listening, whereas before I put cd's on and soon drifted off to doing other things. Now, I LISTEN, and that makes all the difference....have fun!
Doesn't matter....any decision is a good one