There is a group of ECM grounds that on the 2.8 bolts to a machined pad which is located on the pssgr's side of the block, towards the middle bottom. If these aren't connected, it'll run really strange.
On the 3.4, there is no pad, but instead a hole where the crankshaft sensor is installed for crank-fire ignitions. On my 3.4, I snipped the crankshaft sensor wires, left the sensor installed, and connected the ground harness to the single bolt that holds the sensor in place.
If that doesn't seem to be what you're holding, post a picture of the end. But that's the only set of wires that I recall terminating in one big lug like that.
Note if you can't reach that location on the block to connect the grounds, just find another suitable, solid spot on the block to ground them to. As I recall, you may be able to reach thru the rubber splash shields on the pssgr's side wheelwell to get at the crank sensor bolt.
'90 3.4 Troop LS;
'89 Troop RS (Exhaust Valve Challenged), now gone to a Good Home!
Yes, I am a Trooper-Holic!!!
Keep On Troopin' !