I just completed this for my 04 Sport V8 - 75w90 Amsoil Severe Gear in front, rear and transfer case - I've done this on 3 other Tacomas over the period of 4-5 years and this is the first time I've had to deal with a stripped front differential bolt. I noticed that all three had fluid matching in color and viscosity - the front drain bolt had the most metal shavings on the bolt magnet - in the Tacomas the front differential and transfer case are normally clean - the full time 4WD on my Runner must be working good.
My advice is to use "Penetrating Oil" specifically, if on no other bolts do this on the front differential drain bolt, and tap the bolt with a ball and peen or drilling hammer a few times. I used WD40 and no tapping and an already worn front differential drain bolt was stripped. It was definitely torqued at 48lbs + by the previous person who changed the fluids.
To remove this stripped 10mm allen bolt, I simply tapped with a hammer after soaking with Penetrating Oil for 1 hr, used the chisel the make a slit - 2 taps with the hammer and then the cold punch was inserted into the slit and tapped repeatedly, counterclockwise, until the bolt broke free (5-7 taps).
I would also recommend using new crush washers as most others have also suggested, all of my fill plugs had completely flat crush washers - they're only $1.XX to change - do it!
I did use a 10mm allen socket but I also had my 10mm allen wrench handy as I would normally use this to tighten the front differential bolts back down prior to torquing.