oooookkkkkk...
found some good leads...but you got several options...not bad either way...
1)... if you are willing to sacrifice 4mm of length..by shimming something?...moving a few things in your design if possible?...you can use 5th scale hpi baja aftermarket parts. ..they have a 120 mm dogbone setup ... larger stronger cups are also avail...etc...
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXPGP6
and here is my search link for more options like daves..
https://www.google.com/search?ie=UT...e=android-browser&q=hpi+baja+drivesaft+length
2)...a custom shop that makes aftermarket for hpi (and many others)...and most of these driveshafts are produced by these guys anyway...and would most likely make a slightly longer one for you is...company called fast lane machine...and they currently make shafts for the baja up to an inch longer than stock...and they also produce bigger stronger blah blah...lots of parts
http://www.fastlanemachine.net/products.php?cat=Baja+5B
http://www.fastlanemachine.net/proddetail.php?prod=FLM90013
hope this helps...do it dude...