[2000CAYukon]

I don't think your issue is a solenoid. Since reverse works, the sun shell is not broken, the common area for 2nd and 4th is the 2-4 band. If the band is broken or worn, you will loose 2nd and 4th (OD).

In 1st gear both Solenoids are On. For the Shift to 2nd, A solenoid is turned Off. For the Shift to 3rd, the B Solenoid is turned off. For 4th, A is Turned back on.
----1st, 2nd, 3rd, 4th
A is On, Off, Off, On
B is On, On, Off, Off

A bad "A" solenoid would mean losing 1 and 4 (stuck on), "or", 2 and 3 (solenoid will not turn on or electrical issue).

A bad "B" solenoid (stuck on) would loose 3 and 4, or 1 and 2 (electrical).

Since the tranny has relatively low miles, I suspect the anchor for the band broke or the band itself broke. Another possibility is the 2-4 servo is not working. Fluid entering the 2nd gear circuit is fed to the intermediate servo which applies the band. The band is not applied in 3rd gear. Fluid entering the 4th gear circuit is fed to the 4th apply servo which applies the band. It is rare the both seals would go out at the same time. The servo is accessable without dropping the tranny so it is worth it to take a look at. Or have someone take a look at it.

A broken anchor of band will require the tranny to be removed and disasembled.

//2000CAYukon