The 2005 Ford Towing Guide says the Explorer SportTrac is towable 4-down with the Neutral Tow kit installed. It lists no other restrictions but cautions you to check the owners manual or contact a dealer before towing. Does your owner manual indicate a 55 mph limit or is that just something you heard somewhere? There are vehicles with a 55 mpg recommended max while towing, but I wasn't aware that any Ford had that caveat.
The problem in towing 4-down is heat build-up in the transmission due to limited transmission fluid circulation. A speed or distance limitation is intended to help keep temperatures down. The tranny does not immediately break if you exceed the recommendations, but continued excessive speeds or distances take a toll on the life of the transmission. It is smart to check the transmission fluid regularly for signs of overheating, typically a brownish color and perhaps a burnt toast smell. Use of a synthetic transmission fluid will help it cope with high temperatures better.