I have a 1990 jeep cherokee xj, and when I accelerate I hear a real bad clunking and knocking noise and when I slow down I hear something real bad sounding. Anyone know what to do?

 

Coming from where? One of the wheels? The engine? Behind you?

Clunks are often U-joints failing. You're hearing the two big thick pieces of metal hitting each other where the bearings are no longer supporting them. These can be right by the wheel or in the drive shafts. crawl under the XJ and try moving the joints by the wheels and DS. If they move around you'll probably hear a familiar sound.

 

Take it to a mechanic?

 

X2 on this. If you're new to Jeeps, ask a mechanic to diagnose some of the issues and give you a list of what it needs. Then you can go on this forum and see how much work you can do yourself. Usually taking it in for a safety inspection is pretty cheap and then you have a list of the "must dos".