Most likely it is the head gasket, you wont know for sure until you pull the head(s), depending if it is a 4 or 6 cyl. It could also be a cracked head or block, but most likely it is the HG.

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