I must agree with TJKJ, the only time an aftermarket warranty is woth the cost is if it is lower miles, and the only aftermarket parts you add are floor mats and tires/wheels like my wife's volvo. Most of them have a fine print clause that states that any aftermarket modification of the suspension will void the powertrain part of the warranty, and it will list off specific parts.
My advice is to find your original warranty contract, and read it. If it does not say the lift voids coverage, or it does not list the drive shaft/driveline as a part to be voided, then use their contract against them. For them to deny your claim, it must be in the contract as a reason for denial. If they still deny, you may need to balance the cost of the repair vs legal action against them.
I had a warranty like that when I bought my TJ back in '97 (program jeep) and I called to question the clause, and cancelled the same day. Their clause went as far as to say that I couldn't even put mt tires on it, or change the stereo, as that would void the electrical. There are some good AM warranties, but most are the biggest legal rip off in the country, next to insurance of course.
Any way, good luck, and be sure you know what is in that contract you signed before you fight, many count on you not knowing in order to take advantage of you.