I am planning to fly GUA-IAH-PDX next week. Schedule shows two hours between flights. Is that enough time at IAH to clear immigration and customs? I will have two checked bags and will be using wheelchair assistance. There is another flight that has a longer layover if two hours is not enough time. I have never been through IAH (or at least not in decades).

Need more info from you to give precise answer. For now all one can say is that "it depends".

What is your country of citizenship?

Do you have Global Entry?

Will your connecting flight be on the same or different airline?

What day of the week are you flying?

What time are you scheduled to arrive at IAH?

If you are a US citizen and your connecting flight is on the same airline two hours should be sufficient time assuming your inbound flight arrives on or near schedule.

Having wheel chair assistance often can shorten wait time at Immigration and TSA checkpoints as the SkyCap is usually allowed to take you toward the front of any lines. In most instances your longest wait will be for your checked luggage to arrive at the carrousel. That can take anywhere from 10 to 30 minutes.

If you are connecting to a different airline you'll probably want more time just to be on the safe side as you'll have to go to your connecting airline's ticket counter after exiting the Customs Hall to check-in, get your boarding pass and check your luggage before clearing TSA.

Remember Murphy's Law of Airports can come into play here. It clearly states, the shorter you connection time the greater possibility there is you'll encounter delays causing you lots of stress. Conversely, if you have a long connecting time that will virtually assure that you'll speed through every step of the Immigration, Customs, Check-in< TSA process and have lots of waiting time in the airport.

It is United, I think. Probably like these flights:

UA1903, GUA->IAH arrived 8 minutes late at gate E12 today.
scheduled connection 2 hr 1 min to
UA1884 IAH->PDX departed 6 minutes late from gate C1
the estimated arrival time is 9:15, about 12 min from now and 3 minutes ahead of schedule. less than 5 miles left to fly

so, the connection was between IAH gates E12 to C1

yesterday, those flights used IAH gates E2 and C5

Yes, two hours is fine for int'l to domestic connection at IAH, even without the expediting that usually comes with wheelchair assistance. And since there is another flight to PDX later in the day, even if your arrival into IAH is delayed you should be OK as the airline will just put you on the next flight.