Did you understand your mechanic correctly?
This is the method to determine if you have a limited-slip (trak-loc) differential.
1. Place the tranfsercase in neutral.
2. Chock the front wheels to keep the vehicle from moving on you.
3. Jack up the rear of the jeep and support on jackstands.
4. Rotate one of the rear wheels forward by hand. In the case of an open (standard) differential the opposite wheel will turn in the opposite direction of the wheel you are turning. If you have trac-loc however, then the opposite wheel should rotate in the same direction as the wheel that you are turning. You may have to turn the one wheel pretty quickly to get the trac-lok clutch to engage and turn the other wheel.
'95 ZJ Ltd. White 4.0L
TW DC DS; ARB Bull Bar, F&R Air Lockers; Superior Axles; Warn Winch; 4.56 gears; ACOS, Addco F&R Sw Bar, JKS Discos, Rr Track Bar & Nerfs, 249>242 Swap, Stillen rotors+Kolak pads, 3 pc. Armor, RE 4.5 Kit + 2.5"BB, Adj. UCAs&LCAs, OME SS, Bilstein 5150s, 15x8 AR Diamond Backs, 33 Truxus MTs, K&N FIPK, RE 62mm TB, Perf. Distr. 4.0L Kit, Exhaust: 2.5" Kolak exhaust FM 70: Optima Yellow, Mean Grn Alt., PIAA 520s, 540s, Kevins Rack, & TB Conversion