Is the tranny not shifting at all or do you think it's missing 4th gear? These 4 speeds start in 2nd, so you will only notice two shift points. They shift rather early and will have high rpms at highway speeds, but that's normal. To test it, just start in 2nd and shift manually into 3'rd then 4th.
As for the rear end sagging, are the tires bowed in at the top? If so, it could be the hydro compensator on to of the rear axle has gone.
As for the rear end sagging, are the tires bowed in at the top? If so, it could be the hydro compensator on to of the rear axle has gone.