Forklifts are designed to lift and and carry, not push like a bulldozer or track machine. Most often operators force pallots into place by pushing or pulling. If they drag pallots w/ a chain to the rear of a truck instead of using a hand jack, and reverse packs in the trany will ware.
Likewise, pushing pallots tend to burn the forward clutch packs.
This is a 70's vintage truck, whats the previous history? Nobody will know. My guess its time for a trany.