So, went for a ride and put it in 5th gear and now that's where it is stuck.
The gear lever is stuck solid.
Pulled the clutch basket out and tried to turn the shaft that the shift lever rotates and it is definitely stuck solid.
Anybody else had this problem ? and is there a fix other than splitting the cases ?

Never heard of that. What started this, did you crash or hit something?

 

I am thinking if your shift drum is locked up....your gonna have to break it in half.  Its not as big a deal as you might think....may want to get your cheque book ready though..Could be a good time to replace mains / seals as well while its on the operating table. Good luck

Was riding on a road between sections in 5th.

At the end went to change down and it was locked up in 5th.

Didn't drop it or hit anything. 

Was changing gear completely normally till then.

 

If I split the engine is there a gasket/seal kit available that will have everything I need to rebuild ?

 

Most common shifting issue on Sherco would be a broken spring on the selector.  You need to inspect that and find the broken piece before it damages something else.

Probably doesn't apply to your case, but this week I fitted a new gear lever, like the one shown below.  Tried it around the garden.  Seemed ok.  However, when I went to trials last Sunday I was unable to select anything other than neutral or first gear.  On closer inspection the arm of the lever was hitting the gearbox case reinforcement.  I had to reset the lever one spline lower.

http://www.ebay.co.uk/itm/SHERCO-TRIALS-GEAR-PEDAL-LEVER-SILVER-ALL-MODELS-APICO-GPF940-/112548260418?

 

Finally got around to splitting the engine.

Found the problen straight away.

1 tooth had come off one of the gears and was lodged in the slot on the selector barrel.

Ouch. Lucky the tooth didn't get into any of the other gears, would have made a major mess

WoW, now that is something new reported! In the last 17 years, I have only heard of one other broken gear tooth on a Sherco, yet nothing is impossible!