1997 Ford Expedition Rear suspension

About a yr ago the suspension started acting funny. The check suspension light when on and that's when it all went down hill. At first the compressor would turn on and the truck would go up and then lower after awhile. Now the truck's back end is completely lowerd and bottom out everytime you hit a bump or when I pull a trailer the hitch drags. Just wondering if there was anything that I could check myself before shelling $$$$$ to a mechanic in my town that I swear non of them are decent! (Never hear anything good). Please help this single mother that her child is complaining of flying I the air everytime I hit a bump and says mommy has no shocks please get some soon.