Westrunner

I have 2000 4Runner, 4WD, automatic, with 106K miles. Lately it has started "sailing" on me when I let up on the gas - there is little or no resistance from the engine and it just continues at almost the same speed. Definately different than before and causes me to use the breaks more in traffic. This is not a problem now, but I am wondering if this is a sign of worse things to come. I had the tran fluid changed at 90K, before this problem showed up. Any ideas?