I don't actually know whether you can legally sell your fruit. I'm not a lawyer. This might be better posted on the Law SE site. The question might be whether when original plants were purchased they came with a license to sell the fruit. That said, I'll try to address whether the patent is still enforceable.
You don't list the actual patent number so we must speculate. Plant patents, like utility patents, last 20 years from the filing date. Depending on when the patent was filed, which will be earlier than the grant date, the patent on your trees may have already expired. I searched on The Lens which allows specifying just plant patent as the document type. Here is the results for a search of "apple" within plant patents filed between Jan 1, 1998 and Dec 31, 2002. Selecting the first plant patent listed shows US 11519 P, "Apple Tree Named 'jm7'". It was filed on Mar. 19, 1998 and granted Sep. 26, 2000. Since there are no patent term extensions this patent expired on March 19, 2018.
So if you know what specific plant patent is involved, I would search it out and try to figure out when the patent expires. I'm assuming that you are in the US as patents are specific to the country the product would be grown or sold in.