OK it's an 1984 GL wagon FWD 1800 5 speed, and when I decelerate by engine compression in first gear (rather violently stressful on the suspension), the right front wheel makes a loud "thump", and it sounds like someone smacked the car with a rubber or plastic mallet, and you can feel it in the floorboard. But when I get it on jacks I can't for the life of me figure whats loose or broke?!? Please help me diagnose!

Sounds like it may be inside the tranny.Drain the trans fluid and check for particles and such.

Nope, juts a loose tranny crossmember to body bolt was all....PHEW!