[swamp rat]

I would do it soon, even if the rubber is falling away there is a bolt that is holding the cab to frame but without the rubber bushing its putting more stress on the frame and body. Also, there are different body mounts pending which year and model so get the correct bushings.

I replaced mine by using a combination of 2 bottle jacks and blocks of wood, first for a couple day's spray some PB Blaster or equivalent on the nuts to help break them free, then unbolt them, don't forget the front mounts on the radiator core too as your fenders are connected there, lift one side slowly after the bolts are removed, one jack on the cab support with a block of wood, the other on the front, go slow, a little at a time until you can pull the old mounts and put the new ones in, get the bolts back in and start the nuts, then move to the other side.

While doing this you should be looking for any kind of binding, be it shifter linkage or whatever, just be aware.