I had a similar problem when I built a trailer, needed a hitch that would work on
a '90 Carat.  Used a 2" x 4" tube of 1/4" wall, welded it to the stock bumper
brackets in a way that would let it fit under the stock plastic bumper.  Welded a
receiver tube underneath and routed out a small hole to let it poke through.  I
suspect that in a crash, the supports would still buckle, as they are made with
wavy ridges that appear to be a crush design.  The load capacity is unknown,
although the parts I used are similar to those in a 5,000 lb setup.  The stock
supports are the weak link.
The trailer is a 20' x 6' flatbed made for a foot launched sailplane.  It has
hauled septic tanks, my ex wife's furniture, and monster pipe organ parts, without
fail.
    It took a lot of time to build this thing, but it's hard to get exactly what
you want from any vendor unless you are Bill Gates.

    Randy Bergum

Sam Bateman wrote:

> In a message dated 98-11-17 21:05:24 EST, MW575@aol.com writes:
>
> << I went to a local welder and asked about welding something that would fit
>  between the brackets the bumper bolts to, and would fit inside of the current
>  flimsy bumper.  He said he could weld a 2" x 3" tube between the brackets,
> but
>  was concerned about what might happen in a crash to the frame - it could be
>  bent more than it would without such a modification.   The welder said he
>  could do the job for $100, which seems like a cheap insurance policy in the
>  case of a 5-10 mile bump, if it would do the job of protecting some of the
>  engine.  Any thoughts anyone? >>
>
> MW,
>
> On my '87, the trailer hitch that I installed (Valley Hitch) fits inside the
> rear bumper as you described and bolts to the two bumper-support arms that in
> turn bolt into the frame.
> It's cross-member (that fits inside the bumper) is approx. 2" X 2" , 3/16"
> steel tube.
> If I remember correctly, that hitch cost approx. $80.00 from Sears.
> Somewhat cheaper than your local welder and you'd be able to pull a trailer if
> ever inclined.
>
> Just something to consider.......
>
> Sam Bateman
> Fork Union, VA
> '87 Vanagon GL