A local shop should be able to rebuild them for you, if you can't find one, Downey off-road charges about $250. I had mine completely rebuilt/balanced/flow tested for $35 each locally. Have you lost power as well as mpg?, if so, you could have a stuck/burnt valve.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed