If it is a manual trans, which I got from your inference to shfting into gear (5th).. If an automatic, low fluid to a bad kickdown cable.

As long as there is no noise from the drive train, the fix could be as simple as low trans fluid or a loose electrical connection, to having to remove the overdrive and replacing two O-rings for a manual trans.

Automatic fix could entail anything from an adjustment to a replacemnet of the cable.

In an 84, I'd be more concerned with the condition of the wiring harness. Evidence of bad or worse is visible just to the center of the brake booster on the firewall.

Both the above repairs would take a few hours and somewhere near $50 to fix on average.

Wiring harness repair/replacement is another effort in itself. Insurance co rreplaced the harness in the 85 at a cost of $940. I rewired an 83 for $9.00 in shrink tube and intake gasket and four hours labor.

Duane