When I am at a light in drive, the engine seems to idle kind of rough. Any suggestions?

 

Could you tell us more details, year, engine, tranny, have you done a tune up, etc.

 

It is a 1987 Ford Bronco II 2.9 L Automatic. i just replaced spark plugs and air filter.

 

Probably the first check with any idle problem is to check/clean the Idle air control valve (sometimes called Idle speed control).
Second (maybe should be first), run EEC-IV diagnostic tests. Only requires a jumper wire and an analog voltmeter.
Another possible cause of idle issues, especially on this speed density system, is vacuum leaks. Make sure all the vacuum hoses are in good shape.
That should get you started.