[ROVERNET - UK] SD1 (Vitesse) - Manual Gearbox Vibrations on Switching & Replacement of Engine f+r Oil Seals?

[pj.info]

Dear Rovernetters

I did notice since several weeks a strange vibration in the gear lever when 
switching from 1-2, 3-4 or 5-4, i.e., from any odd to any even gear. The 
vibration got stronger and it is now also possible to hear the vibration 
noise during this specific odd-to-even switching (otherwise OK). The 
vibration depends clearly on the speed of the rear wheels and its frequency 
appears to be some multiple of the rear wheel velocity.  

Is there any hint (gearbox lever mechanism problems? / need for taking the 
gearbox apart?)

BTW. How would you replace the front and the rear engine oil seals?

Thank you very much for your advice.

Best regards,
Pierre.