I got one too a little over a year ago in Dallas. My son was driving my '83 Suburban 4x4 6. 2L diesel which has always smoked if you floor it and wind it up some. My letter said the same thing. The engine in my Suburban is an '85 with a little over 100k miles on it. (My truck was stolen & they trashed the original engine at 150k miles).
I ignored the letter. What else can you do. Some people - no, make that most - don't know the difference between an oil burning gasser and a diesel, especially an older model without electronics.