Honestly, the first thing you should do is open up your distribution box. You will be able to see what is coming out of your tank there.
I would bet that a thirty year old+ system may or may not have anything but a "T" coming off the drain line from the tank and the leach field runs off of that though three or four runout lines.
Once you start digging, you may not get the results you want or are looking for, not to mention that damage that may result from a backhoe.
Pumping out the tank is the cheapest and quickest way to find if you have problems, if it is "Full of Shit" then you know that has gone down your leach lines too. By stopping the additional crap running down into them by pumping out the tank, you can try the yeast and see what happens, if it works, then great, if not, then plan on rebuilding the leach field which will "still require" pumping out the tank to solve the original problem of crap draining down into the field before it has turned into a liquid.
I installed a new system about ten years ago when the old system that was here, was full and had filled the leach field and it was past saving.
Nobody had pumped it since it was new from back in the sixties I would guess. I did get a few extra years out of it by having it pumped and then putting yeast into it, but it was too little too late. Cost was about 6,000.00 with new tank and leach field. Couldn't use the old area, was clogged solid and no where to take the bad dirt without paying an additional fee for disposal, was cheaper to install a complete new system at the time.
Basically, I figure start with the cheap end and then go to the more expensive end as required... Just to rent a backhoe to dig for your distribution box that may or may not be there is going to cost more than just pumping the tank