Can't say without seeing seeing up close, but when I see shiny, tacky any sticky-feeling leaves and/or fruit, for me that's usually scale bugs? But I defer to the more experienced...

Thanks Mandarin but I checked for Scale didn't find any. The sap was only on the newly set fruit.

Gro-nut.....More details, please?

Like, how much fertilizer are you using and what kind?
Did you mix in lime when you made it?
Did you spray them for weeks ahead of time before you brought them in?
What kind of Neem?
Have you checked for spider mites?
How long have you been treating for pest before you posted this?
Did you just treat with Neem?
How long is it taking for your mix to dry out between waterings?
Did you ever let it get bone dry by accident?
Did you graduate your plant into your house from outside?
How much sunlight is it getting for how long?
How warm are your temps?
Do you shower it in the tub or even outside on warmer days?
Do you water in sips or let the water exit the bottom freely?
Do you use rain, fresh, or tap water?
What is the pH of your water?
When did you repot?
Why did you repot?
And did you remove all the old mix, bare root it?

Anything else you can provide would help/

Holy questions Mike!!! LOL!! I Use FP@ 1Tbl spoon with vinager and Pro- Tekt about every 10 days or so. As far as sun they get 10 hours a day they sit in south windows,on the other side of the glass is where they are all summer. I spray with Seven each year before bringing the house I suspected Mites that's why I used the neem. I repotted into new 511 the old mix was going on 2 yrs. It's pine bark, turface and perlite. My ph is around 7 so I use a little Vinegar when I Fertilize. I hope this gives you some insite. I haven't had any problems in the past always green leaves and good Fruit. Thanks :)

Gro-nut, honestly, I am not sure..

Ity sounds like you are doing everything correct, except at one time only you may know if you might of caused any kind of shock or damage to your tree or in particular the roots.

Usually with citrus, it can be either, bugs, unseen by the naked eye, past watering issues, either to dry or too wet that can show up weeks after the fact, and or a lack of a good amount of sun and warmth at this time of the year. Light can have a HUGE play in leaf drop, especially if you are getting less sunlight than in past particular years with trees kept at those temps.

Be careful not to over water at this time.

It could also be from the shock of bringing them inside from the outside if done improperly.

I wouldn't worry about it if you have it in a good mix, in plenty of light, keep up with spraying for pest, and watch your watering practices.
If there is any past root damage or shock, it should come around if it is happy at this time:-)

I have found with these trees that leaf drop can be quite common at times and yet the tree will push out tons of new growth to replace the lost leaves, if you keep them happy, especially towards the spring.

Mike