High temp grease, or any other grease should be ok to use, but understand you cannot use it to "Tune" the diffs, that is strictly in the zone of diff oils only as i recently was told.
As to using shock oils, not really a good idea, it works, but not well as it is too thin to be of any use. I tried it in my 1/8 buggy, it was ok but it leaked out from the diff and all over the place after a handful of short runs.
In short, stick to either plain grease or diff oils only. As an aside, NEVER use a grease that is metallic, such as what we brits call "copper slip" grease. It is a grease that it is full of particles of copper and has specific applications, like the splines on the hubs of wire wheels on classic/vintage cars. That kind of grease will do more harm than good if used in a diff.
I actually remember a friend using black treacle once in a diff, good idea, bad in practice.