Sales of tablets with 3G and 4G modems are falling off, according to a new report from ABI Research, an indication that the pricing models for tablet data plans are pushing consumers away from cellular-enabled tablets. According to ABI, less than 27 percent of the tablets sold in the second quarter had a 3G or 4G modem module, down 12 percent from the year-ago period. (ABI said in April that, overall in 2011, 27 percent of tablets shipped with 3G/4G modems). AT&T Mobility (NYSE:T) and Verizon Wireless (NYSE:VZ) are working to counteract the trend with new shared data plans that make it easier to add a cellular-enabled tablet to a smartphone data plan. Overall, tablet shipments reached nearly 25 million units during the second quarter, with total shipments growing 36 percent quarter-over-quarter and 77 percent year-over-year. Apple's (NASDAQ:AAPL) iPad represented nearly 69 percent of worldwide volumes for the period, according to ABI. Release