I have a 2000 awd chevy astro. I recently replaced all four tires and got a recommended front alignment. After driving the van about three miles, the van began to shudder and upon acceleration felt as though some gears were slipping and then suddenly catching. Any ideas?

It definitely sounds like you have something going on in your transmission. If you are not comfortable working on it yourself take your vehicle to a reputable transmission shop. In this situation you are in a dangerous position of likely being ripped off so make sure you trust the mechanic where you take it.
The first thing you should try if you want to take a look at it is flush the transmission and replace the filter refill with fresh transmission fluid. Test, This in many cases like this will have you up and running. If the transmission filter is clogged it will cause the symptoms are are describing when the transmission starts to overheat due to the lack of flow of transmission fluid. Do not run the vehicle until you have flushed it and replaced filter and fluid. Running it in the condition it is in now will only cause further damage.
If this does not remedy your problem you will need to disassemble the transmission and buy a rebuild kit(assuming damage has been done). You could also send your transmission out to have it serviced. This will save you a fortune if you are capable of removing it and installing it yourself. This is the method I use when I have transmission problems. This way you do not have to pay 50 dollars an hour for the easy but time consuming part of the process. You could also buy a re-manufactured transmission. The cheapest but riskiest move is to buy a used low mileage transmission from a salvage yard. Most yards will give you some kind of warranty saying they will replace it if it is dead, but this is not much of an assurance considering the amount of labor it takes to swap and transport a transmission. These are just some of the things I would try if I were in you position. I hope this info helps you out.
George
Feel free to ask if you need some more help.

Sir,

Flush and replace the transfer case fluid on affected vehicles using the following procedure and the part number listed below.

Perform the following diagnosis prior to flushing and replacing the transfer case fluid in order to properly identify the shudder.

Diagnosis

1. Verify the shudder in the vehicle. The vehicle needs to be at operating temperature (driven at least 5 miles) for proper diagnosis. After the vehicle is at operating temperature, drive in a tight circle just short of steering wheel lock, at a speed less than 8 km/h (5 mph) in order to identify the shudder.

2. Identify what rear axle type the vehicle has.

^ If the vehicle has a standard rear axle, then proceed to step 3.

^ If the vehicle has a locking rear axle, remove the front propeller shaft. Refer to the Propeller Shaft subsection in the Service Manual. Drive the vehicle as outlined in step 1.

^ If the shudder goes away, install the front propeller shaft and proceed to step 3. Refer to the Propeller Shaft subsection in the Service Manual.

^ If the shudder does not go away, the locking rear axle may be the cause. Review the owner's records and determine if the rear axle was serviced at the first scheduled oil change. If records indicate that the axle service was done, then revaluate the condition using diagnostic information in the appropriate Service Manual. If the rear axle was not serviced, service the rear axle.
INFORM THE CUSTOMER THAT THE AXLE SERVICE IS AN OWNER'S MAINTENANCE ITEM NOT COVERED UNDER THE VEHICLE'S WARRANTY.

^ Install the front propeller shaft. Refer to the Propeller Shaft subsection in the Service Manual.


3. Flush the transfer case with the blue colored fluid (AUTO-TRAK II) using the procedure and part number listed below. Perform the flush procedure even if the transfer case contains the blue colored AUTO-TRAK II fluid in order to remove any possible contaminants. The AUTO-TRAK II fluid contains an improved friction modifier that should correct the slip-stick condition. Refer to the Transfer Case subsection in the Service Manual for information on draining and filling the transfer case.


thanks
good luck


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The whole engine vibrates is it so?

Sounds as if the engine is cutting out. Check for a glogged feul filter.