My 1991 Ford explorer is swaying side to side what could this be?

First thing for the DIYer to check is tire pressure in all tires. Otherwise seek *hands-on* assistance from qualifying personnel.

Agree with Pushrod. Also would suggest extreme caution. These vehicles were the catalyst for the great Firestone ATX tire recall in the mid to late 1990's - 2003. What was discovered is that the factory recommended air pressure of 26 PSI per the door sticker was a bit under-rated for load carrying and created excessive amounts of heat within the tires -- but was necessary for proper handling. Therefore, be cautious about being above the 26 PSI cold -- and be aware of keeping the load down and ensuring good tires are installed on your vehicle. That vehicle is not known for extreme "stability" as it is. Shock absorbers, steering and suspension components will be important to check -- the vehicle is nearly 30 years old!