Well tow ratings and trans coolers have been covered but lubrication could use some more coverage. Transfer case,engine oil, differential oil, transmission oil, and even power steering oil should also be considered when using a vehicle for towing. My boss told me about a web site that is a forum on lubrication. This is an EXCELLENT site. The first time I spent two hours on it.
www.bobistheoilguy.com
If your into maintenance you'll love this site. Robert