'99 E320 (210) thunk
 

Car emits a solid thunk under strong acceleration from a standing stop.
Engine/tranny mount?
I first thought it was steering as I had a worn tie rod but it still thunks after I replaced the rod.
Any particular procedure to follow to check the mounts?
If one is bad should I replace 'em all or only the bad one?
As far as I know, they are all original.

Will it also thunk if you try to start doing a power brake, in other words if you hold the brake with your left foot and goose the throttle a bit to torque the engine, do you get the same type noise? Sometimes a heat shield for the exhaust gets bent on road debris or some obstruction ans can make noises at certain times when the exhaust hits the shield, as an example. There are ways of physically checking for a worn motor mount. Could be one of those yet, or a trans mount, noises like this are tough to diagnose without being able to look around the car, sometimes if the car is put on a hoist it becomes apparent where the problem is right away to a trained eye.
Motor mounts are typically done in pairs, unless you really don't plan on keeping the car very long. If one is really bad and the other you know is good, you could do just one, in general I'd recommend doing both. Trans mount you can do just that part seperately.

I can feel the thunk when it occurs, it isn't a minor rattle per se. Will try the brake trick.

I have a lift but want to know where the best point(s) are to jack the engine to determine a bad mount.

Oh yeah, popped the hood & tried the throttle/brake trick. The engine was trying to climb out of the bay. New motor mounts for sure.

Any chance that could also be the source of my 55mph wobble?

I wouldn't think so, also does doing the power brake make the thunk noise?