Some disassembly is involved, but not a whole lot. The solenoid side seal is underneath the solenoid mounting plate, so the plate has to come off. That means removing the lever (and losing the adjustment). You'll need a new gasket for the plate.
On the other side, you have to drill or drive out the pin that locates the 'adjustment' lever onto the shaft and remove the lever.
Whether it's possible in situ depends on which car you are working on. On my TR3, I found that I could get the solenoid plate off by undoing the driveshaft and rear mount, then jacking the gearbox up by several inches. Putting it back together might have been problematic, but since I discovered some serious problems, I didn't find out.