IRS private letter ruling

We are a truck equipment distributor that installs certain cranes on chassis rated above 33,000 pounds GVW. After installing the cranes on the chassis, we sell the vehicles at retail. Although we have never obtained an Internal Revenue Service private letter ruling as to whether these vehicles are taxable under IRC Section 4051, the vehicles we sell are virtually identical to those found nontaxable in IRS Private Letter Ruling 199923022. A friend told me he thinks the IRS recently revoked that private letter ruling. Is he correct? If so, how does it affect the tax treatment of our vehicles?