You want an 85 front CV shaft in the rear of your truck. You dont necessarily need the CV up front. The non-cv is going to get more flex and angle than a CV shaft. Most guys who do the SAS or a lift take the front CV (if it is an 84-85) and put it in the rear while using a regular single cardan driveshaft in the front. Most times you are not going more than 25 MPH with the front engaged, so the balance and quality of the shaft is not as important as the rear. Spend your money on the rear shaft. Just make sure the front one is bombproof, especially if you dont have a hi-pinion. A lot of guys build a homebrew shaft out of 1/4" box tube. A cheap and durable way to go, but totally out of balance.

Good Luck!

Kurt G.