Will not go faster than 20 mph - 2000 GMC Yukon

Do you have any codes ? Have you modified your steering/suspension in any way ? Have you changed the size of your tires ? If you can share more detail information, this would help

Generally no. The fastest commercial ship currently can do 67 mph, and a big truck on a suitable road can easily go faster, 75 mph or more.

Check for blown fuse in your van fuse box

Fuel limitation. From tank to carb/injectors are you getting enough fuel supply? also Do you have a clogged air filter? Restricted exhaust system. To test exhaust system, unbolt the exhaust pipes from the manifold and see if it is restricting exhaust. If car runs better it needs exhaust work. All of above is if car runs good under 20 mph, Check fuel filter first, to see if its getting enough of fuel. Good-Day!

It could be a sign of a sensor going bad or the teeth on one of the axles has some that are broken. It might cause an error at slower speeds and clears when you go faster,

do you have aftermarket wheel on the car?? bigger wheels can cause speed signal failure resulting in speedo acting up. different size tires will have different circumference causing speedo issues.

Bring in the car to a PROFESSIONAL shop. Mention your problem & ask about a wheel balance, alignment & make your problem clear before any work is done. Mention bearings etc. Ive dealt w millions of csrs at my own private shop & that would be your best approach. NO JIFFY LUBE TYPE SHOP

definite overheated clanking anything from valves to rods burnt up possible head cracked/warped --driving vehicle when temp high/overheating even at 10 mph will cause internal damage even seize engine if not already

Could be throttle position sensor. Does it run rough at 20 mph or smooth?

This happen to me too. Had to get it replaced under my exstended warrenty that cost me the $200 deductable but would of coast over $400 if this happened later.