This is directly from a Dealers Fact book for a 1989 Blazer (1988 should be the same).
Weight Distributing Hitch Platform Option VR4 (up to 6000 lbs.)
When trailer-loaded weights exceed 2000 lbs., or the tongue weight exceeds 200 lbs., a frame-mounted load-distributing hitch should be used. The platform is bolted to the frame through prepunched holes. Hitch ball, mounting head, equalizing and anti-sway assemblies are also required and are available from outside sources. On V Blazer/Jimmy, the maximum trailer weight/tongue load is 6000/750 lbs. and availability of the hitch platform is RPO VR4.