If the 4 wheel drive option is not working properly on a 1991 Ford Explorer, there might be a problem with the transfer case. The transfer case is the transmission that is used while the vehicle is in 4 wheel drive. It may just need to be oiled or there could be a vacuum hose loose.
The 2003 Ford Explorer has rear wheel drive.
The 2011 Ford Explorer has front wheel drive.
The 2005 Ford Explorer has rear wheel drive.
The 2009 Ford Explorer has rear wheel drive.
The 2006 Ford Explorer has rear wheel drive.
The 2004 Ford Explorer has rear wheel drive.
The 2012 Ford Explorer has front wheel drive.
The 2010 Ford Explorer has rear wheel drive.
The 2001 Ford Explorer has rear wheel drive.
The 2013 Ford Explorer has front wheel drive.
The 2008 Ford Explorer has rear wheel drive.
The 2007 Ford Explorer has rear wheel drive.