3/8" isn't too bad. I just did mine by eye and I know that ain't very accurate but it works. I think i would make it so that the axle is 3/8 offset on the driver side, that way when you hit a bump and the suspension compresses the axle will be moving in the direction that makes it more centered. And go ahead and give it an alignment before you try to drive it again, that may help out alot. 90% of the death wobble I've seen has come from a wallowed out trackbar mount or a loose locking nut on an adjustable track bar. The other 10% would be from the alignment, tire balance, bushing problems.
***EDIT: Now that I think about it, it seems weird that just one turn on the tb would move your axle a full 3/8". not sure what tb that is but it seems like you got something else messing with you.
