Beckford and Joe Garner had already put the tie beyond the visitors with strikes either side of half-time before Beckford grabbed his second in the dying stages with an audacious effort from 50 yards.
It was tough on brave Chesterfield, who had a golden chance to go 1-0 up 140 seconds before the Beckford opener but Sam Clucas missed when through one-on-one.
Preston will take on either Swindon or Sheffield United in the final. The pair meet on Monday night with Swindon leading 2-1 from the first-leg.
Friday’s meeting between these teams was heated affair and that aggressive tempo continued with Garner and Ian Evatt involved in a couple of feisty exchanges early on.
Garner was booked for a connecting with a swinging arm on the defender before Evatt got slight revenge minutes later by going through the back of the striker as they challenged for an aerial duel.
Garner would eventually get the better of that battle for the opening goal but Chesterfield really should have taken the lead on 35 minutes.
Gary Roberts worked the ball through to Clucas who was only 10 yards out when his effort deflected off on-loan Manchester United goalkeeper Sam Johnstone’s foot and over the crossbar.
Beckford showed Clucas how it should be done just two minutes later.
Garner did brilliantly to rise and win a header against Evatt which found Beckford in the box with his back to goal. He still had a lot to do but finished with impeccable ability, hooking the ball over his shoulder and into the net.
Chesterfield defender Sam Hird had an effort cleared off the line just before the break by Paul Gallagher as the visitors tried to stage a rally.
However, Preston always looked a threat on the break and won a penalty via that method of attack on 60 minutes.
Daniel Johnson raced clear in behind the Chesterfield defence and Joe Murphy came rushing out to deal with the danger in the corner of the penalty. He was slightly late and slid in on Johnson, who had nicked the ball away from the goalkeeper and the referee pointed to the spot.
Garner stepped up and with one swing of his right foot hammered the ball into the top corner, ending the tie as a contest.
Jay O’Shea fired against a post on 75 minutes but the last word would be for Beckford, who scored an outrageous effort.
He broke free on the halfway line and spotted Murphy off his line. From just under 50 yards out the former Leeds man, despite slightly slipping, pinged an incredible effort, into the top corner to send the home fans wild. A fitting goal to send a team to Wembley.
Watch the second League One play-off semi-final second leg between Sheffield United and Swindon Town on Monday night on Sky Sports 2 HD from 7.30pm, and the League One play-off final live from Wembley at 5.30pm on Sunday 24 May.