Thank you for everyone that offered ideas. Here's what I figured out.

My garage door opener is a Craftsman Model 139.53992 €“ ½ hp screw-
drive.

I disengaged the trolley latch from the screw drive and tested the
door. It moved freely without binding. It would stay fully closed
and opened without assistance. If I opened the door 1/2 way up and
let go, it did pull back to the closed position. If I opened the door
2/3 the way up it would balance and stay stationary.

I then disassembled the housing to look at the mechanics. As a future
note to others disassembling this opener, to take the housing off
you: (1) Open the light lens. (2) Remove bulbs. (3) Remove the two
screws holding the cover to the bracket €“ these are in the upper
corners of the bulb area. (4) Pull the cover *forward* (toward the
lens) and remove. If you pull it downwards, youll brake the tabs on
the rear plastic piece (like I did). It just wouldnt be a home job
if I couldnt use duct tape in the repair. Check that one off. �

I was surprised at how simple the mechanism was. There are two gears
on the motor. The first is a gear that engages a belt which then
drives the screw. The second is a worm gear that engages the limit
switch assembly.

I immediately noticed the gear on the limit switch assembly was barely
engaging the worm gear on the motor shaft. I could gently apply
pressure to the limit assembly gear and it would €œjump€� downwards and
turn. Neither gear looked to be stripped or worn. However, both
gears were dry as a bone.

I also noticed if I applied very gentle pressure to the bottom of the
assembly (pushing upwards) the gear would fully engage. I removed the
assembly and cut a slim cardboard shim (from a 9v battery package).
There are three holes in the main bracket where the limit assembly
€œclicks€� into place. I put the shim in the bottom most hole and
inserted the limit assembly back into the bracket. The gear now fully
engaged and I could not force it to €œjump€� down and turn.

I didnt have any heavy grease, so I grabbed a tube of silicone and
lubed up the friction points on the limit assembly and where the
assembly engages the worm gear on the drive motor. I hope this wasnt
a mistake. Is regular silicone compound OK to use in a situation like
this? The door opened this morning (8+ hours after repair).

I appreciate everyones help and advice. Hopefully this will save
someone else a repair call in the future. If it ends up this is not
the fix, Ill post an update.

Thanks!