4266foster Burgh

I was given a 1994 Grand Cherokee 4.0 withe the 42RE automatic that was left behind. Overall in good shape, looks like someone put in a new or rebuilt transfer case and possibly transmission as they are both pretty clean. Front driveshaft is removed.
Vehicle starts and runs fine. Put it in reverse and it really grabs hard. Drives perfectly fine in reverse. Go from there to Drive nothing, L2 a little pull, Low It drives but feels like it is fighting itself almost like brakes are on.
If you go from Park all the way to Low and skip past the other gears completely it drives in low normally without the fight. If you go back to Reverse and let it engage at all and then back to low it fights itself again and struggles to drive forward.
I checked Trans oil, proper level and red. But noticed that the previous owner was putting Dex MErc in it and 1994 calls for Mopar not sure how much that matters.
Also checked to make sure that the brakes arent grabbing in some way.

Any suggestions of where to start?

oddballracing

The best place to start would be to try to flash out the codes (if any) from the TCM, being an electronically controlled tranny and all. I recently purchased a '95 ZJ for 'parts' due to a transmission issue encountered by the previous owner where the rig was trying to take off in 3rd gear in the D range. i was getting codes 21,23,22 and 55 (in that order). 55 is normal, it just signals the end of codes, but the other three are all errors thrown regarding the control solenoids on the valve body.

To flash out the codes, you'll have to have had the transmission engaged (drive, reverse or otherwise) in range since the battery was connected/charged so the codes will enact.

Starting with the key OFF, you'll want to move the ignition key ON-OFF-ON-OFF-ON without engaging the starter, pausing about a second in each position. The codes will flash out sequentially through the O/D OFF indicator lamp to the right of the steering column (at least that is where it was on my '95, should be the same for '94). I had to try a couple times as there is no two-step flash-out (slow for tens, and fast for ones) like i was used to on some OBD-I implementations I've seen before. These will flash out the total code number, then pause and then flash the next code, until you get 55 (end of codes).

Once you have that information, then the next steps will be to determine what may be throwing the code (it may not actually be the part referred to by the code, so don't just throw parts at it right away - solenoids and transducers can be pretty expensive).

Here is the fault code list for the 42RE:

dave1123

Get the Dex/Merc out of it ASAP! That thing is supposed to have ATF+4 in it. I think the FSM for date on manufacture says ATF+3, but 4 supersedes 3. Using anything else will cause clutch chatter or slipping. Also, the 42re is very fussy about fluid level. It should only be checked in neutral, not Park. The torque converter isn't pressurized in Park and that can make a difference of about a quart. One quart low can make it shift funny.

4266foster Burgh

Thanks for the good infomation I will check it out and let you know what I find.