We have a whole section dedicated to the transmission cooler failure inside the radiator:
http://thenissanpath.com/viewforum.php?f=38
To summarize it, there were a lot of radiators on North American, 2005-2010 Pathfinders/Xterras/Frontiers that experienced a seal failure of the transmission fluid cooler located inside the bottom tank of the radiator. The radiators were factory installed units made by Calsonic and they had a sticker on the top tank with one of these part #'s: 21460-EA200. 21460-EA215, 21460-EA265...and occasionally we run across a failure with a later number, 21460-9CA2E. Not all of them failed, but, there were a lot of them and many of them resulted in engine coolant contamination of the transmission fluid and damaging the transmission.
Nissan extended the warranty and then, again, after they lost a class action lawsuit in NY state. The extended warranty covers the radiator and subsequent damages for 8-years/80000 miles at full coverage, then cost limits of $2500 and $3500 for 90000 and up to 100,000 miles, respectively. If you have a 2008, you are most likely past any warranty claim.
A failed radiator and trans can result in a repair of $5000-$7500 in most cases. As a preventative maintenance option, many have replaced the radiator or bypassed the radiator's transmission cooler. In bypassing, the cooler lines are re-routed so that the transmission fluid passes only through the factory auxiliary cooler in front of the A/C condenser. It 100% guarantees there will be no chance of trans fluid/engine coolant contamination, but, sacrifices some cooling ability and the ability of the radiator to warm the fluid and maintain it in the optimal temperature range of around 175-200 degrees F. Most of those who have bypassed and monitored trans fluid temps usually report see ranges around 150-165 degrees, but I have not heard of anyone having any issues with running the bypass even with moderate towing. Some have installed additional air-to-liquid trans coolers, but it seems that this is overkill.
The other option is radiator replacement. There have been no cooler failure issues with the aftermarket radiator options that I've heard of to date nor of the Nissan Value-line radiator, which is usually in the $350 range. If I were towing heavy loads or lived in an area of extreme temperatures, I would recommend replacement. If $350 does not sound out of line, Stillen offers a CFS all-aluminum radiator that is pretty nice! Plastic-tanked radiators, like the factory unit, are available for a lot less. You can find generic units for around $75 on Ebay, but many prefer to stick with a name-brand like Spectra Premium or Koyorad for a little over $100.