motor problems
 

1990 f250 4 wheel standard 5 speed
Motor is a straight 6 300
I took it out in the mud today and got stuck. A friend of mine hooked up to my hitch and it took us 10 mins of jerking and me hammering on it everytime to get free.
When I got out I noticed the trucks idle was bouncing from 1000 rpm to 500 rpm up and down. On the way home when the truck was in gear but no load coasting it was sputtering but soon as I put the motor under load it goes away and acts normal. When I got home it was almost dark but could see the exhaust was what looked to me like light black.

Any ideas where to start trouble shooting?