88 F350 4X4 Brakes

Have an 88 F350 4X4 with weak brakes. The truck is low mileage and has been pretty well cared for. Anytime I hit the brakes a little hard, the rear wheels lock up and skid. Doesn't feel like the front is doing much. I disconnected the rear plug to the anti-lock brakes and it seemed to help a little. I'm going to go ahead and do a complete brake job front and rear but I'm not sure this will correct rear wheel lock up problem. If anyone has any advice, things to check or change, I would really appreciate the help. Thanks in advance.

1988 Ford F350, single rear wheel, 4X4
7.5 liter (460 ci ) gas V-8
Auto Trans, 9000 GVW
rear anti-lock only

rear axle seals???

That indicates that the rear shoes are not retracting well, or that the parking brake mechanism is frozen.

Fortunately, Ford went to outboard mounted drums in about '86, so you do NOT have to pull the axle shafts and hubs to inspect the rear brakes on your '88 (unlike GM that is still inborad mounted - idiots).

A simple rear brake job and repair of any frozen or worn parking brake components should solve this.