[jveik]

what exactly do you mean the tranny bogs down? just that it slows down? if this is the case, it's probably not too big of an issue. just dont let off the gas and it wont slow down. im assuming that you have a turbo 400 or turbo 350, (probably the turbo 350) both of which are 3-speed automatics.
your bogging down effect is likely just the fact that these are only 3 speed trannies that cause you to be spinning at probably 2500 or more rpm's at 60 miles an hour. when you let off the gas, the motor just tries to return to idle rpm and is in effect engine braking. i used to drive a truck similar to yours, just it had 2 wheel drive and i loved how you could slow down quickly on the interstate by just letting off the gas and letting the engine slow ya down.

of course this explanation only applies if you have a feeling that it simply wont coast or something like that, if it is really slowing down bad, and it has never done this type of thing before, like you were slamming the brakes or something, then i have no clue what it could be...

if it is having trouble keeping up speed at part throttle, it may be a carburetor problem. i tried to switch out a carb on my truck for some other carb and it would only run strong at full throttle.

is the tranny slipping or anything, like it revs and then catches or whatever?