[RockyMtnRay]

Originally Posted by Gateway

I have a 2001 Chevy Express 1500 with the 5.0L V8 and 135 inch wheelbase.

The 5.0 makes 220-hp @ 4,800 rpm & 280 lbs.-ft. @ 2,800 rpm
It has a 3.42 axle and the stated towing capacity is 5100lb
the trans ratios are:

1st-3.06
2nd-1.63
3rd-1.00
4th-0.70 (OD lockout is recomended for towing)

Chevrolet reccomends brakes over 1000lb and distributing hitch with sway control over 4000lb.
I have pulled our 12x6 enclosed cargo trailer(3000lb) with no brakes without any problem.

I'm looking at either the 3124ks or the 3326 king. Would these be within the capacity of the van? Also I may replace the van with a 2004 with the 5.3 V8 @295hp/335lbft which puts the towing capacity upto 6200lb.

You're borderline on torque to pull a loaded 3124KS and especially a loaded 3326KS. Both trailers will weigh between 4500 and 5000 lbs when fully loaded. Your 5100 lb "tow capacity" is measured with the van totally devoid of cargo or passengers...just driver and 3 gallons of gas in the tank. The weight of all cargo and passengers in the van have to be subtracted from the "tow capacity". If you, for instance, fill the fuel tank (~120 lbs) and carry 700 lbs of passengers and cargo in the van (a reasonable number), your real "tow capacity" will be more like 4300 lbs. Exceeding your vehicle's limits is a very poor/dangerous thing to do. I concur with the Bob that your biggest limitation is the 3.42 axle ratio.

Bottom line is the van will manage (though with poor acceleration) to pull either trailer on flat, low altitude roads such as in the midwest. It will struggle if you try to tow in mountainous regions and struggle very badly in high altitude mountainous regions such as the central Rocky Mountains. Expect to spend many miles climbing grades at only 20 to 25 mph if you try to tow across Colorado.

Replacing your van with the newer, more powerful model would be exceedingly prudent.

Finally, you absolutely will need a trailer brake controller for the trailer's electric brakes and you will also need a WDH (just like your owner's manual states). Sway control on the WDH will not, however, be needed.