Tires

A 100 load index is good to 1,764 pounds per tire.
The 102 load index up to 1,874 pounds per tire.
So yes, the 102 would be more than the front.
I found that your vehicle weighs 4,306 lbs, so they should be okay.

I just want to make sure it is okay to mix index sizes on my van.

Yes, it should be okay.

I would say no, it is not okay to mix tire sizes on that vehicle. The load index is determined by the size of the tire so you are trying to mix sizes. You car has ABS brakes and having different size tires will mess up that system and your differential.

He was talking about the load index, not tire size!

I am aware of that. Different load index means a different tire size.

I did not know that. I suppose you are correct.

There are exceptions but as a rule the load index and tire size go together. The exceptions are optional extra load tires and sometimes speed rated tires that are only made in limited sizes. He really needs to tell us the sizes to confirm.

I agree!

225 /65 /R17.

Are both choices that size?

Yes.

Do all the tires have to have the same speed rating? I am a speed limit driver, and the fastest I will drive is seventy mph on the PA Turnpike.

Okay, it will not be an issue as long as you are replacing all four. You would not want to mix them as they might perform differently.

Speed ratings effect a lot more than speed limit. The construction of the tire is entirely different so you need to stay with at least the recommended speed rating and do not mix them.

I had to replace the front two a couple of weeks ago, but I just need the backs, and I am having trouble finding 225 /65 /R17 100 Index S rated.

It is not ideal but you can get away with two and two.

I checked the front ones that are new and they are marked 106H. Never heard of 106 and I cannot find a retailer with 106's.

Now you are getting confusing.
You started out asking about 100 and 102, then you had 100 on the front. Where is the 106 coming from?
What size and speed rating did the truck come with?

The ones that were on the van when I got it from my parents were 225 /65 /R17 102H. The new ones say 106H. The new ones are Nokian Entyre 2.0 model. I thought the fronts were 100S, but I was mistaken.

You need to be more concerned with the H speed rating than the load index. The truck came with H rated tires and needs to stay with them. If you put a lower speed rating and have an accident you are opening yourself up to a lawsuit.

So I really only have to look for the H while price shopping?

The only advice I can give you is to replace the tires with the exact rated tires it came with. I cannot advise you to use a lower speed rating or load index. Higher is okay.

Higher of which one is okay? Both?

Either or both, but FYI, S or T is not higher than H.

I do not know what the original specs of the tires that were put on at the assembly line. Right now the front has 106H and the rear has 102H.

It is printed on the door jam.

The size on the tires matches the size on the pillar. The only thing not printed on the sticker are the tire rating and load index.