While under my car for an oil change, I noticed that the rubber in my torque strut mount is badly cracked. (The picture shows how dirty it is, but not the cracked rubber.)

I don't notice any new issues while driving, but because it'* only $9 at RockAuto, I'm thinking I should just replace it. Is it a difficult swap?

I would replace it ASAP. Can't tell you how difficult it would be since I've never owned that year Bonneville myself, but a broken mount will put unnecessary stress on the remaining good mounts, and can cause other problems as well due to the excessive engine movement in the engine bay.

i've certainly seen worse...

The pictured dogbone is held in by two bolts. IIRC they are both 15mm'* or 13mm'*. It'* actually a very easy swap.

Oil and grime will degrade and break down rubber BTW. So it'* a nice thing to keep the engine bay clean.

That picture you posted is for a torque axis mount. That one seems to be quite intact and not leaking. The one that'* breaking is the torque strut mount, which is down near the oil filter.

Not that I don't like visiting you, but is this something I could do on my own (i.e., without an engine lift)?

The top half of my engine gets cleaner every day, thanks to my recent discovery of brake parts cleaner. My transmission case is so shiny now, you'd think I finally replaced it. Bottom half is next on my "I'm bored today" list.

odd.... alldata doesn't show any other engine mounts for a bonneville....

From what I gathered by reading other threads, it'* this one, Anchor part #2881.

It'* very easy to do. Take the two bolts out. Slip it out. Pop the new one in and tighten the bolts.

No engine lift or jack needed? I don't want to pull off the old mount, and then need to push up on a quarter-ton of drivetrain to slide the new mount in.

Nah.. it'* a two bolt thing. Just like I said.