So you have been married for > 4 years. You would have to apply for Entry Clearence as a Spouse and you would receive a limited leave to enter the UK with a "KOL Endorsement Required" which means you will need to take your Life in the UK Exam prior to reapply for indefinate leave to remain in the UK again (if you so desired). I forget the new rates, but we are talking at least £550 for the entry clearance and £850 for the new ILR.
Of course you could travel to the UK as a visitor, take your life in the UK exam and then submit that in the state for your entry clearance and you would receive an indefinate leave to enter (same as an ILR) and you only have the one application fee. This is an important concession that our very own Garry and UK-Yankee worked to get available to folks.
If I am not wrong it is technically discretionary for the IO to invalidate the ILR upon re-entry after two years (at least that is what the letter I received with my ILR said that I "may" run into issues). If you are just coming back for a visit, I wouldn't worry about it. If you are coming back to settle, I would apply ahead of time.
The tough thing is that all the rules are changing and you could have in theory applied for your Citizenship after 3 years of living in the UK when you were here before and not had to hassle with all of this. You may need to wait now 5 years or perform community service in order to be eligable for your Citizenship this time around. *sigh* Welcome back!