Is “pass peach seeds” an idiom or just a figurative expression?

To pass peach seeds is a more polite version of a country expression, to shit peach pits. Since peach pits are large, hard, and sharp at each end, passing them through one's anus would be exceedingly painful and would likely cause some damage requiring surgical repair.

The expression means something painful, and feared because it is painful.

I have heard this expression all my life. Although it is picturesque, I think it is also hyperbole, therefore a figurative expression. A peach pit would be likely to perforate some soft tissue in the gut long before it got to the anus to be passed.

Edited to answer a question from the comments: Where I am from will not answer your question as to where this expression is common. I heard it in Georgia, USA (nicknamed The Peach State) and in orchard areas of Iowa and Washington State. My husband, who grew up in Texas and Montana, also uses this expression.

To shit a brick as someone else said is a different phrase, one with equivalent crudity but without the witty image of the peach pit going through the digestive tract. A person may accidentally swallow a peach pit, but how is the brick going to get in there?

The meaning is 'to eliminate a peach seed from one's system by defecating it' (or, to put it more vulgarly but concisely, 'to shit a peach seed').

Here, the verb 'to pass' is a euphemism for 'to expel from the anus', which is probably familiar to many English speakers from the expression 'to pass gas', i.e. to fart.

The implication of this allusion is that for some reason Jeff was finding his situation as uncomfortable as if he was expelling a peach stone.

Incidentally, most native speakers of English would probably refer to peach stones or perhaps peach pits rather than peach seeds — although botanically, they are indeed seeds.

Pass: to discharge (body waste, for example); to be voided:

Luckily the kidney stone passed before she had to be hospitalized. (TFD)

It's a polite was of saying to "evacuate" (in these cases, from the gastrointestinal tract.) If you've ever seen a peach pit (the seed of the peach), you might imagine the discomfort associated with passing one, let alone multiple pits.

You should have seen my man Jeff waiting for me outside the liquor store. He looked like he was passing peach seeds.