Modify IFS/CV joint for more travel:
- http://www.4x4wire.com/toyota/tech/cvmod/

If you have enough slip length (depends on how much front suspension travel you have), then a driveshaft spacer would do the trick:
- http://www.4crawler.com/4x4/ForSale/DriveshaftSpacer.shtml

Otherwise have a shop make the shaft longer w/ a long travel slip yoke. And yes, is possible to use ag. spline in a shaft, but it'll be a relatively loose fit as those splines are made for relatively slow speed (PTO) operation.